Building an engine tutorial

Added Appendix B to the first post. Reposted here as well.


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

Is that for sure, particularly the last bit? I had an impression that larger, slower cylinders in the general are more economical.

Is that for sure, particularly the last bit? I had an impression that larger, slower cylinders in the general are more economical.

Cylinder size doesn't always relate to speed. The optimal speed of your engine is a different speed from maximum power speed, and different from best torque speed, and all that. The best is to go for balance unless you actually need one over the other, like in a tug or a generator.

They key here is that larger sylinders with a slower running engine are more economical compared to faster running engines. Remember fuel is injected, in a 4-stroke, every two revolutions of the crank, so if you have two engines of the same total size and one has half the cylinders but they're twice as big, and it runs say at 3000 rpm, that's fuel being injected 1500 times a minute. If the smaller one runs at say 8000 rpm fuel is being injected 4000 times a minute. Even if you push it so far to say the larger slower engine will inject twice the fuel (Never happen) that's still less then the faster one by a bit.

Yes, that's why slow engines are more economical. But why would many small cylinders be more economical than few large ones?

Yes, that's why slow engines are more economical. But why would many small cylinders be more economical than few large ones?

For same speed, less fuel. Most people reading this, I predict, are going to have an engine for a relative speed anyway, like a car at 8000rpm, or a tank with 3000rpm, so it works better to generalise and point out that way. There are more complicated ways to go about it, like differeing RPMs, compression, combustion effeciency, the design of the valves themselves, all can drasticly change your fuel economy. But I'm throwing out rules of thumb for people, not full blown text books, I'm too lazy for that.

Just what's the prime cause of it? Engine imbalance, better mixture combustion in small volume, something else? Or if it's empirical, any specific example?

It's volume of the fuel. You will always need more fuel (and more air) in a larger cylinder to get enough energy from it to work. There is a minimum energy level for the engine to sustain itself, the larger the moving parts (and more friction) the higher that level, and thus the higher the ammount of fuel and air that needs to be injected to combust.

But in a many-cylinders engine of same displacement you need to make work more cylinders, don't you? And the total friction AIUI is higher since there is more total surface.



[Not considering switching cyls off, that's hightech outside the basics].

Yes, and no. Those do compound the issue for many cylinder engines, but again, volume. I don't know the engineering maths behind it, but I do know, larger cylinders = more fuel used. It's pretty universal.

anyone heard of Sleeve Valve 4-stroke engines? (like the Bristol Hercules and Napier Sabre), Diamond Star use 4-stroke Inline 6 or V12 sleeve valve diesels

17 to 32 litres in capacity depending on operation, most trucks will however be sold with 17 to 19 litre V12 engines, which like the Magirus - Deutz engines used Older Iveco and some Tatras are Air Cooled

Are all opposed piston engines sleeve valves? So it'd be an accurate assumption to say they have high oil consumption rates?

No.

Do you have any examples? The ones I'm finding seem to be two strokes where one piston slides past an intake port at the bottom of the stroke, and the other piston slides past the exhaust port.

http://en.wikipedia.org/wiki/Opposed-piston_engine