Building an engine tutorial

The Quick Guide to designing a realistic and capable engine.

I thought it might be worth writing a quick guide to designing a piston engine for those of you out there who might be interested. All too often people will just assume they can add another thousand horsepower to a tank and be done with it, but it's not that simple. This guide will hopefully show you how complex an engine is, and at the same time guide you in designing the simple statistics to your own. My credentials, for those of you who may be curious, is a family history of maritime diesel use, my father, several uncles, and many many people I grew up around being marine diesel mechanics. I've had the luxury of hands on experience with these, especially high output, engines for many years now. I hope to pass this experience to you through this guide, at least as much as possible.

Step One: What do I want my engine to do?

This is probably the single most ignored step in all of NS for a piston engine. Piston engines, especially diesel, are temperamental and need to be custom tailored to specific tasks. Thankfully custom tailoring is rather easy, and a well designed base engine can be adapted to a wide variety of applications. But it is important to consider what use you will have for the engine in question, as they will all have issues. A marine diesel for a fishing boat will be different from one for a frigate because the fishing boat will need constant low speed power for days if not weeks of non stop running and little maintenance, a frigate on the other hand will often need speed while having the ability to stop the engine for extensive maintenance. Automotive gasoline engines will need to be small and lightweight, and need to use the commonly available fuel stocks, while still being reliable, which will cause them to suffer in terms of power. I will be concentrating on an engine for a tank for this example however, because this is where you are most likely to see the need for a piston engine on NS, and where most people are likely to mess up.
Now that we have decided what we want the engine to do, we know what we have to work with, or can figure out what we have to work with, in terms of space, power, speed, reliability, and fuel consumption. The engine for this example will be a fairly conventional engine to replace the MTU 870 series engine in a Leopard 2.

Step Two: How will my engine work?

This is probably the most crucial step because it will dictate everything. The first thing we need to consider is how the engine will cycle. What this means is we need to decide if this will be a two-stroke, four-stroke, or one of the more exotic designs like a six-stroke. They all have their advantages and disadvantages, all will greatly impact how the engine can operate. Some important thing to consider first is power stroke. The number of power strokes per cycle, in plain and simple English, is how many times the engine goes boom per times the piston goes up and down. Every stroke is approximately half a rotation of the crank shaft, meaning one stroke is the piston either moving up, or moving down. Now that you know what that is, we can move on to describe the different cycles.

The four-stroke cycle, also known as the Otto Cycle, is the standard cycle for most engines, especially in cars, airplanes, and tanks. The four stroke is a little less powerful then others, but makes up for it in much less fuel use. For most normal sized engines this is probably the best conventional approach to use. The four-stroke is a ¼ power stroke engine, meaning one out of every four strokes is a power stroke, the remaining three are used to draw in the fuel-air mixture, compress it, and eject it from the cylinder. This is good because it allows a more efficient combustion, and thus less fuel escaping unburned in the exhaust. The down side however is that the engine has to rotate two times for every power stroke, forcing the use of multiple cylinders for balance, and reducing power per RPM, or rotations per minute of the crank shaft. Four stroke engines typically have to run at a higher RPM to get sufficient power.

The two-stroke cycle is more commonly used on very large diesel engines, such as oil tankers, or very small engines such as model aircraft engines or lawnmower engines. The basic principal of the two-stroke is simple, the first stroke starts with the combustion of the fuel-air mix, forcing the piston to move down, which opens intake and exhaust ports on the cylinder walls and allows fuel-air mixture into the cylinder and exhaust out, while the second stroke compresses the fuel-air mixture and combustsit. This operation is very simple mechanically, and thus very easy to build small or large, but it has some serious problems. The need to use intake ports in the cylinder wall and one stroke for both intake and exhaust means that fuel will always end up unburned in the exhaust, and exhaust will always remain in the cylinder. This is bad because it means less efficient burn of the fuel-air mixture and very messy exhaust. The bonus of the two-stroke system is that it is a ½ power stroke engine, meaning that it has twice the power strokes per full rotation of the crankshaft as a four-stroke engine. Most two-stroke engines also operate with the fuel actually being a fuel-oil mixture, forcing the fuel air mixture into the cylinder below the piston during the compression stroke so that it lubricates the crankshaft and piston components before being combusted, which saves a lot of weight in oil systems but means that because of the added lubricating oil in the fuel, that combustion efficiency is even worse then normal. The bonus of using this, besides lubrication, is that the motion of the piston going down can be used to force the fuel-air mixture into the combustion chamber above the piston with more pressure, which better forces the exhaust out of the cylinder. Some tanks use this system, including the 5TD and 6TD engines in the T-64 and T-84 series tanks. They're usually very lightweight, powerful, and small, but more costly on fuel then a four stroke and generally very dirty.

Now earlier I mentioned the exotic six-stroke, so I will elaborate on it only briefly. There are several six-stroke engine systems out there, many working differently from each other. The basic idea however is the inclusion of two more strokes to the cycle to change a four stroke engine to a 2/6 power stroke, or 1/3 if you wish, instead of the ¼ power stroke of a four-stroke engine. I would generally recommend that you avoid these systems unless you know and understand the rest of the process first. After a little playing around with two and four stroke engines however, you may be interested to take a look, probably the most useful of the six-stroke engines however is the Crower cycle, because it is the easiest and most powerful. The others all have their places however, and might be worth some fun later on.

For our example engine we will be using the four-stroke system. We want range, decent power, and commonality with the vast majority of other systems around, as well we want to be a little environmentally friendly.

Step Three: Ignition, how will the fuel burn?

This may seem like a simple question to answer, but in fact it's not quite as simple as it may look. How the fuel is ignited in the engine will dictate what fuels the engine can actually run. We have two basic ways to take this, compression ignition and spark ignition. These are two differing camps in how to run an engine because they are essentially mutually exclusive. Gasoline and other similar fuels such as most jet fuels, require such a high pressure to com bust through compression that it is impractical to design an engine to use this system if gasoline is to be used. On the flip side, gasoline combustion under compression is significantly more efficient and powerful, but as stated, it will put exponentially more stress on the cylinder and piston, which means the engine will be significantly more likely to fail catastrophically. Diesel, however, is much much easier to burn under pressure, but doesn't flash light very well when sparked. So diesel engines will use compression, not have a spark plug, but instead they will have the force of the piston compressing the fuel-air mixture to burn. Gasoline engines will need to be sparked by an electrical ignition, a spark plug, to be used however. As we want a multifuel, and as we won't be running too high compression ratios in this engine, we will opt for compression and instead bulk up the engine to compensate for when we need to use gasoline. This means the engine will be heavier, quite a bit heavier, but it will allow us to fuel up on any commonly used engine fuel.

Step Four: Layout and displacement?

This is both one of the most important and most useless steps of them all. The easiest part to tackle is layout. Do you want an inline, a vee, a boxer? Maybe something more exotic like a W-engine or H or even an X layout. They are all essentially the same in terms to how it will effect the engine workings. Of the above the only thing really effected is the physical size of the engine. An inline engine will be long and narrow because each piston will be in a line. A vee engine will offset the pistons such that it is wider but shorter, and a W engine will take this even further to make a wide but short engine, but will limit the displacement of individual cylinders, and an H or X engine will be tall, wide, and short, but complicate externals such as fuel and intake/outlet systems. The vee engine is most common because it is the easiest to produce and maintain in the smallest package, and we will be using a vee engine for this. However, the other options we have, that I will discuss breifly here, are the boxer and one I didn't mention, the opposed piston. The boxer engine is essentially a vee engine but flat, so that cylinders are opposite of eachother. This removes the need for counterweights on the crankshaft of the engine and allows it to run smoother because the pistons balance eachother. This can increase power, but it does have the issue of being very wide. An opposed piston engine is entirely different all together. In an opposed piston engine you have two pistons, with two seperate crank shafts, sharing a single cylinder. This as many upsides, not the least of which being the ability to run higher compression ratios because the combustion pushes on two pistons instead of a piston and a head attached with a few bolts, allowing more pressure for less failure. Opposed piston engines can also get more power per cylinder of a given dimension because smaller cylinders still mean more movement of the pistons, allowing much more power from less displacement, which I will touch shortly. The major downside to the opposed piston engine however is increased complexity. The need for two crankshafts on opposite sides of the engine means you need to join them through a centrak gearing system before putting into a transmission, and of course it doubles the mechanics internally around them, for this reason they are often two-stroke engines.

Displacement is the single most important factor to how much power you can produce then anything else. American hot rodders have a saying that goes “no replacement for displacement” and this holds true everywhere. In short displacement is the internal volume of all the pistons combined at their maximum capacity. This is really just a combination of bore and stroke, which is how wide a diameter the cylinder is, and how long it is with the piston at the bottem of it's cycle. More displacement means more air, means more power from each deatonation, and means less wear on the engine because for a larger bore you need thicker cylinder walls. You can alternativly go with longer strokes and narrow bore, but this will actually suck power from the engine as the fuel will burn so quickly as to only act a certain ammount. Higher power engines typically are short stroke but wide bore diameter engines to increase power but decrease time between power strokes. Displacement is the hardest thing to change in an engine because it will require a complete redesign if it's more then a small percentage of the engine. So, rather then take you through all the details of how to make it, I will just wave my hand and decide we will be using a 48L displacement engine. This is a good average size, the MTU 870 series engine of the Leopard 2 for instance is 40L displacement, and will give us some nice power gain without seriously increasing the size of the engine. Another aspect of this that I will touch before we move on is number of pistons, because it is related to displacement. But first, MORE PISTONS DOES NOT MEAN MORE POWER. Repeat that to yourself several times until you have it memorized. More pistons means a lot of things, better balance, smoother exhaust and intake, more power strokes per rotationof the crank shaft. What needs to be considered most is that the displacement of the engine is the sum of the displacement of the individual cylinders, and thus to a point more cylinders allows you to make them smaller for more displacement. Ferrari uses this quite effectivly for instance, with the 4.7L V12 engine of the Ferrari F50 having about 0.4L per cylinder, so each cylinder is small, moves much shorter distances, and can be cycled quickly. For our engine we will have ten sylinders because it's a good number to keep the engine ballanced and vibration free, while at the same time allowing slightly larger cylinders for easier combustion of lesser fuels. But in reality we could go with a V10 or V8, or anything, it doesn't matter as long as the displacement is enough. So for now we have a 48L V10 engine running primarily on diesel fuel, but with multifuel capability.

Step Five: Inductuion and exhaust?

Now we are going to start to get fancy, but also complicated. So for ease I am going to address this in order, induction, exhaust, and then “other” for a few specialty things to add in.

Induction is simply how the air from outside the engine gets inside the engine to be burned. It is important to know that more air means more power, when you press the gas pedal to your car you're not really putting more fuel in the engine you're putting more air into the engine. So this is the part where you can get the most power. Here you have really two primary options, natural and forced induction. Natural induction means your engine will suck the air in itself and require no assistance, this is less powerful unless the engine is very carefully designed and maintained, but has the upside of being signifigantly less maintenance intensive. The other choice is forced induction, which means in short that air is forced into the engine by a compressor. Two basic types of this are the supercharger and the turbocharger. Now without going into all the different sub types of each I will just tell a basic outline of them. A turbocharger uses a turbine in the exhaust to turn a compressor that forces air into the engine. This takes no power from the engine itself, and can give a lot of air to the engine, but it has problems. The need to use exhaust means that you need to have high pressure exhaust right out of the engine to run the turbine, as well because of the excess ammount of air forced in turbocharger engines have to run at a lower compression ratio then naturally inducted engines. A final problem is what is known as turbo lag. This is means that the turbocharger does not deliver at lower RPM or initially, the engine must be kept running fast to keep enough exhaust pressure to run the turbine, and so a turbo is useless at lower speeds. Because generally the larger the turbocharger compressor, the larger the power boost, this inversely means that the larger turbocharger will take even more exhaust pressure to provide any boost. There are ways around this, such as using two or more smaller turbochargers that spool up quicker but provide slightly less power, which is the most common way of doing this. Another, not as common, approach is the use of a smaller turbocharger combined with a big one, so that the smaller unit will provide limited boost at lower RPM and allow the bigger one to spool up quicker, this helps but isn't seen as often because of the steep power curve it causes. Another alternative is the use of the turbocharger along with a supercharger, which gives the best of both worlds, provides a flatter power curve, but has some serious problems. At least one production car and race car has used this system before, but it is otherwise rare unless aftermarket installed. Although it is quite a bit more common on marine diesel engines.

The other forced induction option is what is known as the supercharger. It's essentially the same as a turbocharger except that instead of using a turbinr driven by exhaust gasses to run it is driven directly off of the engine itself. This has the downside of sucking a small ammount of power from the engine, but the upside is that at lower RPM speeds it provides substantial boost. Once RPM speeds get higher though the supercharger begins to suck more power then it provides, or even begin to suck air from the engine itself, and as such needs to be considered differently.

For exhaust the only think you need to really think about is if you're going to have a turbocharger. A turbocharger needs the exhaust to be run through the turbocharger turbine, and that means it may need to be routed oddly, causing an unnessecarily long exhaust system. Other then that, as long as the exhaust can freely move out of the engine and out of the exhaust system, then you are set.

The last part I will mention, as “others” here, are some things to include on this track. Air coolers, for instance, which cool the air down at various stages of the intake process. Cooler air is denser air, and thus more of it can be packed into a cylinder for more power. The most common cooler would be the intercooler, where air from the forced induction system is cooled before being put into the engine. Forced induction has the odd habit of heating the air it compresses, making it less dense at the same time, so an intercooler will change that to increase power.
Another thing to consider is fuel additives. Specifically water-methanol, propane, and nitrous oxide. They all have their uses and advantages, as well as disadvantages, but shouldn't be considered for industrial engines. I'll touch them breifly here because I know someone will try to install “NOS” on their tank engine at some point, and this will help teach you why that is a bad idea. N2O, nitrous oxide (Never “NOS”, that's a brand name of a company that produces kits for aftermarket add on to cars) is a system where quite simply N2O is injected into the engine. This syetem won't work with diesel engines, it will only work with gasoline and to a limited extent pure ethanol. N2O works two fold. First it drops the air temperature of the intake where it injected, meaning again denser air, and second durring combustion it breaks down and delivers more oxygen to the combustion process, which again adds more power. Because of the pressures involved though, this doesn't work with the relativly low pressure diesel combustion.
Water methanol works in a simmilar way, but is ideal with forced induction engines. What it is is essentially a combination of water and methanol, and is sprayed into the turbo or supercharger to prevent air from lighting under high boost compression in these systems. This means more boost, more air, more power.
Finally is propane injection, which is the counterpart to N2O in that it is the diesel counterpart. Injecting propane into a diesel engine will cause a more efficient burn still of the diesel combustion, and will give a little more power, and unlike the other two systems can be used constantly, but for much less power increase.

For all of this talk we will take our 48L V10 and add an intercooled twin turbosystem, with the option for propane injection to be used later if we need. This will give us a highly efficient engine with good reliabiity and power for a decent size. Now for statting it out.

Step Six: Good, now how much power do I have?

Now we get to the fun part. After we've understood all the parts going into the engine, and have a basic layout we can work with, we can begin making our stats. This is more guesswork then real math though, as every engine is different. But here I will give you the basic figures to know things that might be inportant to you. First we will cover power, the one everyone wants to know. To know power we must know the optimum RPM for the engine, for our example this will be a nice round 3000 RPM. Gasoline engines will need higher RPM to get more power, but diesel engines will typically opperate best in the lower speeds, with huge two-stroke engines of ships sometimes running as low as 94 RPM. But for us 3000 RPM will give us a good balance to run anything we need to while still getting power from our four-stroke system. A general rule of thumb here, appart from looking at real engines, is that the higher the displacement the lower the RPM, and this holds true for both gasoline and diesel engines. Lower RPM also means less wear and tear, and thus less maintenence. But how do we get power from this? This is where handwavey comes in. Power is a combination of more intracate things in the engine then I can detail to you in this guide, but a basic rule of thum to estimate it for diesel engines in the same application and setup as what we have is that you can get 20-35 horsepower per liter normally, 40-60 horsepower per liter with a turbocharged or supercharged engine, this is for these medium sized ones. And it is important to remember that the more power systems will need signifigantly more complex, heavier, and larger systems to get that power. These ratios are generally the best way to approach how to determine power. But remember that as engine speed increases the engine will make less power on the same ammount of displacement, but as engine size decreases it will make more power for the displacement, but this will rise less then the power will drop with speed. Smaller faster engines will need more complex systems, especially forced induction, to increase power, but will be able to do it much easier then larger engines because less volume needs to be compressed before being forced into the cylinder. Conversely lower speed engines will be less capable of producing the same ammount of forced induction boost pressures as faster engines, and larger ones will have more space to fill and thus need more intake and boost.

So, for our engine we have decided most of the important specifications, and now can work out the power. We'll go with a rounded number within the bracket we can have to make it easy, so we'll arbitrarily choose 2000 hp. It is important to remember this is engine power under ideal situations, and that hotter or cooler air outside will cause this to go down or up, as will altitude, and any number of factors. But as a good baseline we will go with 2000, which is realistic. Since we opted earlier, for fun, to install propane injection if we want, that can go up to 2100 with propane injection, but no higher.

So what about dimensions and weight? Well, engines are typically built out of steel and thus weight a lot and are big. You have two ways to go about doing this, first you can take the bore diameters you may have decided on earlier and add them up. If we have a bore diameter of 170mm then each cylinder plus walls, plus cooling and other systems will take uo about 200mm of diameter. So six of them, the length of a vee engine, will be about 1.2m. But then the additional parts on the front will add another 20-30cm, so the total length of the engine will be about 1.5m for easy rounding. These numbers can be played with to an extent, so it should be taken more as a guideline then anything, althougn don't go too radically different from them. And remember, reducing the ammount of surrounding space on the cylinder increases maintenance needs because there is less room for wear to happen before the cylinder fails completely. Width is a little more complicated, and depends on some weird math knowing the angle of the piston rods in the vee engine, and the stroke length. For inline engines this is just the width of the previous bore ammount, 200mm, plus about 20-30cm of additional stuff to the sides. So that an inline may be 40cm wide. A boxer is easy because it is just the stroke length plus 30-40cm. A vee however to get accurate you need to know the angle of the vee, which we will be lazy and say is 45 degrees. This way we can basically say if our stroke is say 200mm long, then our width from the centre will be 100mm, and so the total stroke length will be 200mm, plus 50-80cm of additional externals. Height will be done in a simmilar way to width, keeping in mind the height of a boxer is the width of an inline, and the height of an inline is half the width of a boxer. Vee engines are typically square in their sizes of height and width, or sometimes taller or sometimes wider, depending on stroke length. We have a short stroke length though so we'll do with a shorter engine height. Final rounded statistics then will be 1m wide by 1.5m long by 0.8m tall. This will give us a nice compact engine that is very capable. The other way to do this would have been to find an engine of simmilar displacement, layout, induction and speed, and copied their stats rounded as needed.
And then we need to do weight, which is much easier to do now because I will provide you with some numbers. First you can simply copy the weight of a simmilar sized and shape and laypout engine, or you can follow this. An average excellent power to weight ratio for the most modern engines is between 80 and 90 kg per liter of displacement. Going higher will cause serious problems, and older engines will be much lower. So our engine will thus weigh about 4000kg now.

The final performance aspect to consider is torque. It's not as important in NS because no one thinks of it, but it's worth a glance. Torque is a factor of engine speed and power, meaning the slower engines will produce more torque and the faster less. All I can really tell you without taking you through some nasty math is to keep the torque number around the power. So our 2000 hp engine will have abour 2200 lb-ft of torque, with roughly the same when propane injected to keep things simple. Now the final part.

Step Seven: The stats, yay!

This is the shortest, last, and best part. We're going to write our stats down, and we'll do it in a simple way. The simple stats we need are just this:

Configuration: V12 Twin Turbocharged
Displacement: 48L
Power: 2000 bhp
Torque: 2200 lb-ft

We can however expand this if needed, or include this information in write ups for fun. An expanded list will look more like this.

Confugiration: 45 degree V12
Induction:Forced, twin turbocharged
Displacement: 48L
Bore/Stroke: 170/150
Length:150cm
Width: 100cm
Height: 80cm
Weight: 4000kg
Optimum RPM: 3000
Power: 2000 hp
Torque: 2200 ft-lb

Now there are all kinds of things we can discuss further such as fuel consumption, cooling systems, and other things. But they're not important to our needs, all that needs to be remembered is that these things exist. Bigger engines eat more fuel, more powerful engines and faster engines need more cooling capability. But for the short of it all, this is everything needed to design and implement an engine in detail or in simplicity in a design. And it may help you better understand how your own car works.
If there's anything I missed or that you want to have elaborated on, feel free to ask.

Appendix A: What not to do when building your engine

Ok, this is a breif add on that I have decided to add because I see these things happening all too often.

Don't assume all engines are equal.
This is straight forward, unless the engines you are looking at are doing virtually the same thing as what you need, then don't bother using them as a reference. A racing engine and a tank engine are completely different, and the characteristics of one will not carry over to the other's application. Especially with the case of racing engines which are fine tuned for a specific and very short service life. In theory you can stick a 1000 bhp racing engine in a tank, but it will last you all of an hour before it destroys itself. This is bad, very bad. Decide your application, find engines to assist you looking over from that application, and work from there. Don't mix-n-match, it simple doesn't work that way.

Don't turbocharge a Crower cycle.
For the same reasons it's a stupid idea to turbocharge a steam engine, you can't turbocharge a Crower cycle engine. Turbochargers need hot high pressure exhaust gasses to work, the whole injection of water into the cylinders completely ruins this and means your turbos actually reduce horsepower by restricting exhaust flow and intake flow, and add weight to the overall engine. Not to mention the water itself is not good for the turbine.

Don't add 1000 bhp to an existing engine.
Don't, simply don't. Most modern engines for use on anything but giant marine applications are so finely tuned to get the power they have now that you are virtually maxing out their potential power. You can not take the engine out of a Leopard 2 and add another thousand horsepower, you can't. You can't even take it and add five hundred horsepower without serious consequences. Yes the engine is rated at a maximum output of 1800 bhp, but running an engine at maximum rated output means you are running it into the ground and will have to replace it very very often.

Don't assume you can make an engine drmatically more fuel effecient.
Because you can't. An engine will run with X ammount of fuel injected per cylinder per cycle every time. That simply does not change. More or less air is added to create more burn and thus more or less power and speed inside the engine, but the fuel remains the same. And that ammount of fuel is decided by the displacement of the cylinders. The larger the cylinders, the more fuel injected. Thus the more power the more fuel used. The faster the RPM, the more fuel used. The fewer the number of cylinders per a set displacement, the more fuel used.

Don't mix engine types.
I've already explained the different engine layouts, but I want to make this clear anyway. You can't apply the same type of charactaristics to a V or H or I engine that you would to any other engine. Modifications made to an Opposed Piston engine, or even the general characteristics of it, do not apply to a Boxer engine. If you need to understand this please re-read the section above regarding that subject. But remember, they can't simple be changed from a B12 to an I12, or an Opposed 6 to a V6. Especially not with the likes of the Opposed Piston engines and other special ones like rotarys, radials, and other non-conventionals.

Appendix B: Real Life fuel/range ratios for modern tanks.

The below ratios are divided up into weight classes based on combat weight, and are taken only for MBTs which have a power/weight ratio at or above 20hp/tonne. Provided data comes from Janes Tank Recognition Guide, and includes stated range (Where nessecary the lower of two estimates is taken), fuel capacity, and from them their ratio.

50 tonnes or above:
- LeClerc (550km/1300L): 2.36L/km
- Leopard 2 (550km/1200L): 2.18L/km
- Arjun (450km/1610L): 3.58L/km
- Type 90 (400km/1100L): 2.75L/km
- Khalid (400km/950L): 2.38L/km
- M1A2 Abrams (426km/1907L): 4.48L/km

40-50 tonnes:
- Degman (700km/1450L): 2.07L/km
- T-80B (450km/1100L): 2.44L/km
- T-90 (550km/1200L): 2.18L/km
- M-84 (700km/1450L): 2.07L/km
- T-84 (540km/1300L): 2.4L/km

30-40tonnes:
- AMX-30 (500km/970L): 1.94L/km

30 tonnes or less:
- TAM (940km/640L): 0.68L/km

Well, one thing I see...

It is possible to use nitrous oxide on a diesel engine, in fact it's fairly common in the world competition diesel vehicles, sometimes in addition to water-methanol injection or propane injection.

Also, though, if one is intending to use nitrous oxide on an engine, its a good idea to have a secondary fuel system to add extra fuel to the engine at the same time as the nitrous is being injected, to help keep the air/fuel mixture in the ideal range.

You'll see why I said it's not for diesel's when you said "sometimes in addition to". N2O doesn't give enough of a power boost to warrant all the added weight without needing even more extra stuff, not with a diesel at least.
In fact I'm pretty sure diesel N2O injection doesn't work at all without Propane injection already installed to bring the burn effecnency up.

I also wrote it that way to discourage people from using it, because a lot of people will go watch sopme stupid movie like "The fast and the furious" and think they can apply it to say a tank engine. Outside of piston powered propeller engines for aircraft, putting N2O injection on a military vehicle is a stupid thing, it increases wear, increases weight, and adds a small ammount of power that's limited by the ammount of space it takes from your fuel capacity. Civil engines for cars and racing are different, but that is by far more rare then the tanks always being designed. This is here to correct the people who believe adding 1000hp to an engine is easy, or claim PMT as making it easy.*

It all comes down to the same reason I didn't elaborate very much on forced induction. I could have explained the differences between supercharger types, clutching systems, and all that, but why bother? I'm the only person who uses a roots blower on a tank engine, and I'm probably the only person who uses a dual turbocharger-supercharger system.

*By the way, what's written here applies for all times. You won't get signifigant power increases from 2050 tech over 2000 tech if you're using a piston engine. We've reached a limit at what we can do and without going to a completely new design such as a rotary or a gas turbine, you're screwed to the old "no replacement for displacement" mantra.

Now that I did some more research, it appears the intent for using nitrous oxide on a diesel engine isn't exaclty the same as it would be with a gasoline engine.

Evidently in modifying a diesel engine for racing, it's fairly easy to cause the engine to run excessively rich. One way to help counter this is by adding a nitrous system, which adds more oxygen and brings the air/fuel mixture closer to normal. This makes the burn much more efficient, which itself adds power to the engine, combined with the cooling effect on the intake and lowered exhaust gas temperatures.

So it is possible to run nitrous oxide without propane or water/methanol injection on a diesel engine, but it is only practical within the confines of competition engines where outright power is the most important thing. This is entirely not the case for a military engine, because you wouldn't need to balance out excessive amounts of fuel.

A Breif guide to APU Boosting:

It occured to me today that APU boosting is one of the most widely ignored, but potentially important aspects of an AFV drivetrain, so I decided a breif guide to it might be in order as a tack on to this one. So let's start.

What is APU Boosting?

In short, APU boosting is the practice of using the auxilliary power unit (APU) to boost the time to power of the engine. It takes several forms, and operates differently depending on the engine used, but generally speaking it is designed to bring engine to it's optimal power quickly. Now you may say to yourself, or me, "Well Sumer, you're an idiot! Engines always have power!", but you'd be wrong. Engines operate with a power and torque curve, which means that unless the engine is at it's peak power within a narrow part of it's RPM limit. Tanks and other AFVs often use the transmission to adjust the actual speed, and keep the engine within the peak performance part of it's powerband. Generally speaking, the flatter the powerband, the better for performance. There are cases where this is not the case, but I'm not entirely sure the disambiguation matters for NS. Now there are two ways to flatten the powerband, that is to make it's peak, or close to peak power output cover as much of the engine RPM spectrum as possible. One way, which has seen use in racing, has been to provide the engine with both a supercharger and a turbocharger. This was discussed above, and was the primary reason one of the arguably most powerful and fastest rally cars ever designed was banned completely from competetion. The problem with it however is that it leads to plenty of mechanical issues, especially with diesel engines which are lower speed then simmilar sized and powered gasoline engines (Although lots of marine diesels use it), and is completely useless with gas turbines. So the second option is APU boosting, which can be used as long as their is a sufficient APU system installed under armour (If it is an AFV).

How does APU boosting work?
There's a few ways it works, but I'll start with how it works with a typical turbocharged diesel engine in most AFV applications. In simple terms, the APU is used to boost the exhaust pressure and temperature so that the turbocharger turbine which is in the exhaust system, spinns up faster. This essentially eliminates turbo lag (As described previously) in that the APU, which should already be up and running, is providing the exhaust pressure to get the turbo spun up. This means that the APU has to be fairly close to the engine however, and definetly has to be within the armour of the vehicle. Gas turbine based APUs are ideal for this, as they provide the most exhaust gas pressure for the size, but they eat fuel quickly. A small piston or rotary APU however could provide the needed exhaust quickly with a well designed exhaust system, albeit slightly slower then the gas turbine. It's important to remember however that this doesn't do anything for engine start up time, it only improves the engine's time to power. While an unboosted MTU 870 engine (the series out of the leopard two) may start and run in two to three minutes, it will take six to eight minutes more to warm up to power depending on the environment. APU boosting the engine will still take two to three minutes to start up and run, but within that time frame it will have quickly warmed up to it's peak power because the turbocharger will be feeding it tonnes of air from the start.

But how does it work with gas turbines? Well, it does and it doesn't. A well designed APU system can be used to force it's exhaust gasses into the gas turbine and help get it spinning up faster, which will reduce start time. The problem is that either the gas turbine has to be small, or the APU has to be big, otherwise you won't be seeing much of a reduction in start time. There are alternatives, such as the system I believe being included in the M1A3 Abrams with the UAAPU inclusion. Basically, the idea is to use the APU to provide power to a seperate forced air induction system to feed the primary gas turbine. In simple terms it's a turbocharger for the gas turbine powered by the APU. This has it's upsides and it's downsides. Firstly, what it does is forces more air into the gas turbine faster, which reduces the time it takes to spin up and get running. The problem is that the faster the gas turbine spins up, the less the effect of the APU boosting, and in fact after a certian point the APU system will just restrict the air intake of the gas turbine, reducing it's power. However, if used for startup, it should reduce fuel comsumption for the startup process of a gas turbine, and bring it to power speed quickly.

There is a breif introduction to APU boosting, I hope it helped.

I think the Block III for the Abrams (M1A3) was canceled in the mid-90s. The M1A2 will be introducing a lithium-ion battery whenever this battery is ready for production. But, AFAIK, it's less for APU boosting and more to reduce fuel consumption of the AGT-1500 when the engine is idle, and to allow the tank to use the sensors when the engine is off. I know your write-up (really informative, by the way) is specifically for engines and the APU's for engine boosting, but I just left this as a note. The Leopard 2E uses an UAAPU, but I don't know exactly what it uses - I think it's electric, according to SAPA (the manufacturer).

Ill be using this when I design my MBT, thanks a lot!

Well, earlier I've mentioned that (as only natural for a nation with dominating aerospace and no ground vehicles) we mostly use turbines, but now I've finally found something which would need diesel engine. However, not any will do...

Do you know of any type, or something, to make an engine very resistant to water? The engine is likely to operate with air intake very low, possibly damaged, and so it's inevitable that water will splash in the intake quite often, and the air will be very humid anyway. Of course, diesels already provide that to the extent, but I'd prefer to make it even better in this regard.
This will be complicated by relatively small size, lack of space around, and lack of qualified personnel - it's for a lifeboat/rescue boat. So it also should start easily, and be quite reliable, for its week or two of use. In case of choking, it also should tolerate it somehow, not necessitating sucking air from the inside.

So, suggestions?

I would suggest taking a look at the MTU diesel engine being used on the Marines' EFV project.

MTU 883? Yes, the power/weight seems good. However, that engine operates in pretty soft conditions compared to lifeboats, and only for a small time to bring the force ashore, so I'd rather just borrow the solution making it compact.
While MTU 880 series are quite tolerable, it probably offers no improvement in reliability over water-cooled engines already used. Well, and, besides, the point is to make an engine design, so I just don't want to take RL ones. Solutions and ideas to make an engine more tolerable to seawater are more interesting.

A normal marine diesel would be fine. Rule of thumb for continious use and heavy wear on the engine is to make it double the displacement that you would have in a truck or tank for the same power. So, if it's a rescue boat then something simmilar to the engines in fishing boats should do fine.
If it's going to be thrown away after no more then two weeks use, then you don't need to worry about underway maintenence as much, but if it's going to be used for years then you want to make sure it's durable.

Edit: Volvo has your aid if you need generals.
http://www.volvo.com/volvopenta/global/en-...s/diesel_range/

Cooling is generally done through pipes that run outside the hull in the water, so it's not an issue of radiators, and those engines are designed to be run in really wet conditions. If you've never been in a fishing boat or smaller commercial boat like that they are generally very wet.

Well, most times it won't even be used apart from exercises. Not really two weeks overall, but about a month total lifecycle, with the actual escape being the last, if it happens. It's a rigid self-launching lifeboat to be stored aboard to let the passengers or crew escape, not a standalone rescue boat.

That's, effectively, a tiny craft (10 tons or so), operated in open sea through storms. Issues involved are quite considerable; I don't want to use interior space as air buffer due to uncomfortable and dangerous effects involved. Well, I guess you remember that scene in Das Boot where the engines choked; it could actually rip the eardrums on some subs, and be worse in event of a lifeboat, if it's given a good engine.

So the point is that the engine shouldn't throw any bad trick when the intake is splashed with water, and skip the cycles, but continue working when the intake is clear.


Edit:
Well, I've been on small motorized sailing yachts (not really have much time, but I enjoy sailing), so know how wet it can get. However, not this wet, when the thing will actually often be completely "submerged" in a wave. And it should improve the resistance further than usual. Normal lifeboats tend to use interior space as the buffer, though sometimes being separated, but it's somewhat space-consuming for a high-output engine.

Yeah, of course, it's to be water-cooled.

You don't need high output then.
Best bet is to have a seperate engine compartment, nothing too big, just like a "sealed" box in the middle of the boat or something that can be opened for maintenence. Air intakes and all that will be designed to have various valve systems and splash protection so that anything but complete submergance will not hurt it. Complete submergance is going to kill the engine any way though.

Now, it's about the way to make it survive almost complete submersion... for a moment, but still. Either by not letting water into sensitive parts or by tolerating more of it than usually.

Output has to be not high, but considerable, due to worse conditions here, where it might turn out in wrong waters and need to move by itself some distance, before the supplies aboard run out. The buffer might get damaged. Furthermore, there's no space to make an entire compartment, just a section below the double bottom.

So, is there a way to increase salt water resistance, or just make it live through some temporary lack of air?

Nicely done Sumer! Im highly impressed, and although i thought i knew the basics of piston engines this has enlightened me further. Definitely useful, i hope you don't mind if i use some of this info on my version of Mac's Nakil 1A1 upgrades, i wouldn't mind having something slightly more powerful in my version, plan being i redesign the hull in line with something like the Nakil 1A2 without the hull based rounds...should give enough room for extra fuel to keep range the same for the more powerful engine.

So thanks, good job and i look forward to using it in future!