Compression ratio

[WhiteBlue]

Our engines cylinder stats:
Bore x Stroke / Volume
27,7 x 70 mm / 42 cm3
That's a quite a bit longer stroke than usually. Do any gurus know why we have do it this way?

I suspect that your engine runs much lower rpms than F1 racing engines and that stroke to rod ratios are also much different. Slow revving long stroke engines produce more low power torque as you see in tractors or ship engines. Your engines are made for ultimate efficiency. So the assumption is that they have a good thermal and internal aero efficiency as well due to the slow speeds. I would be surprised if your engines run more than 1,000 rpm. Probably they are even under that.

[marcush.]

engines is not my strongest point but i feel the compression ratio you are capable of running is limited mainly by your ability to avoid engine knock .

with a small bore longstroke it is a lot easier to achieve serious compression ratios with compromising piston crown shape ...but as your nore gets bigger the shape of the combustion chamber will ineviatbly be flat and big area..and thats not the best starting point to achieve good flame propagation and good mixture distribution in the chamber...
My A-series (austin)historic race engines all had 13:1 compression ratio with point type ignition...as long as you had your cooling temps as low as possible
and paid attention to detail in piston and headpreparation that was really workable on 100ROZ shell v-power.
the piston crown in such an engine of course cannot be flat...and you have to really look into cooling fluid flows

[diosol]

Our engines cylinder stats:
Bore x Stroke / Volume
27,7 x 70 mm / 42 cm3
That's a quite a bit longer stroke than usually. Do any gurus know why we have do it this way?

I suspect that your engine runs much lower rpms than F1 racing engines and that stroke to rod ratios are also much different. Slow revving long stroke engines produce more low power torque as you see in tractors or ship engines. Your engines are made for ultimate efficiency. So the assumption is that they have a good thermal and internal aero efficiency as well due to the slow speeds. I would be surprised if your engines run more than 1,000 rpm. Probably they are even under that.

You made few common misconseptions. Long stroke doesn't mean great low torque because it's result of something else. Our engine starts at 1000 rpm and goes up 4000 rpm and we would use it even higher but it vibrates too violently. F1 engines that have opposite rod ratios than ours have higher efficiency rate than we eco-marathonist have even though they dump fuel to exhaust and rev to 21000 rpm. Now that's awkward!

[WhiteBlue]

It all depends of how you define efficiency. There are horses for courses. And you cannot compare vastly different courses as F1 and a fuel marathon.

[diosol]

It all depends of how you define efficiency. There are horses for courses. And you cannot compare vastly different courses as F1 and a fuel marathon.

I don't know what efficiency did you mean but when speaking of engines I think of how many procents does it make power of fuels energy resource and how many procents does it goes to waste heat. It's the same even for ships 2-stroke diesels. Please, define me your view of efficiency.

[mep]

ah by the way:
My colleague also claimed that high revving (F1) engines have a higher efficiency because less energy goes into cooling due to the high revs.

This is something you don't take into account in the thermodynamic calculation because you could not use a isentropic processes then. I thought you do so because the effect is very small because of the fast cycles.

[riff_raff]

The geometric CR of an engine is not so important. It only serves as a convenient way to validate an engine's static dimensions. What really matters are cycle dynamic pressures and temperatures (ie. P1, T1 and P2, T2). Dynamic CR can also vary widely depending upon RPM, cam timing, trapping efficiency, etc.

High RPM race engines, with large bore/stroke ratios tend to be relatively inefficient with regards to BTE. This is due to the fact that friction losses in the engine tend to increase exponentially with RPM, and also that thermal losses are made worse by the extreme large surface-area-to-volume ratio that exists in a large bore/short stroke race engine combustion chamber.

In theoretical terms, increasing CR (and of course Expansion Ratio) will improve thermodynamic efficiency. But in practice, a CR of about 14:1 has shown to be the best compromise for ultimate efficiency, for both diesel and gasoline engines. While 14:1 may be OK for a gasoline engine on race fuel, it usually is not practical for a compression ignited diesel, since you would never get the diesel to fire up. The practical CR limit for most small bore diesels is about 16:1 or 17:1 due to startability.

riff_raff

[WhiteBlue]

It all depends of how you define efficiency. There are horses for courses. And you cannot compare vastly different courses as F1 and a fuel marathon.

I don't know what efficiency did you mean but when speaking of engines I think of how many procents does it make power of fuels energy resource and how many procents does it goes to waste heat. It's the same even for ships 2-stroke diesels. Please, define me your view of efficiency.

If you compare what arrives at the wheels of the car in terms of power as a fraction of the fuel consumption I would agree. Because you have to substract all engine internal parasitic consumption and the transmission losses from the raw work the engine does.

[riff_raff]

If you compare what arrives at the wheels of the car in terms of power as a fraction of the fuel consumption

I would somewhat disagree. IC engine efficiency is usually defined in terms of brake thermal efficiency (BTE). That is the amount of power produced at the crank (or dyno "brake") divided by the LHV of the energy content available in the fuel mass combusted. Or in simple terms of gas pressure work applied to the piston, IMEP-FMEP=BMEP.

If you consider the efficiency losses (mechanical + aero drag) produced by the chassis, the engine BTE numbers become fuzzy. An F1 engine has somewhere around 35% BTE. Which means that 65% of the chemical (LHV) energy of the combusted fuel (about 18,600 Btu/lbm) is sent out the exhaust pipes, lost to engine mechanical functions, or transferred into the radiator airflow, and the rest is available at the crank to move the car.

riff_raff

[strad]

You've left out VE which goes down as rpm goes up.

[n smikle]

One of the things I know is that an engine with good miles per gallon does not necessarily equate to an engine with good thermal efficiency.

The long 70mm stroke and tiny bore of disol's engine.
I think of this.

Small bore:
Low heat transfer to the side walls.
Low friction at the side walls

Large stroke
Less gas reversion due to gas being drawn away from the valves
Less acceleration on the C-rod bearing

The compression stroke takes less power too, because of the small bore. So At low throttle cruise speed, the engine pumping losses go down. I tihnk.

[strad]

I believe, You want the rod length to stroke ratio close to 1.5:1 to get the crank throw at 90° so you're not wasting energy pushing on the crank at the wrong angle.

[n smikle]

Yes a long rod is more efficient, but he didn't give any information on the length of the connecting rod.

[strad]

It's not length it's the ratio

[autogyro]

It's not length it's the ratio

A very important point strad