High compression turbocharged engines.

[Kozy]

I have seen some builds of naturally aspirated engines, originally using around 10:1 CR, being built up to 13:1+ CR and then having 15psi of boost thrown at them from an aftermarket turbo kit.

I can't comprehend how this is possible without the engine melting, what do you need to do to make a combination like that work without detonating? I can see how it might work with direct injection and an ATDC ignition point perhaps, but on an old port injection engine? Baffled...

 

[Ranger Mike]

nascar engines ran 14 to 1 CR...modern drag engines run 16 to 1

check out followign article...excellent reading

http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/

 

[Kozy]

nascar engines ran 14 to 1 CR...modern drag engiens run 16 to 1

At 2 BAR MAP?

 

[Ranger Mike]

yep...not too outrageous back then since stock compression rations were 13 to 1..
the 16 to one thing i am not to up on, hence my link to the Pro Stock article..
bumping to 14 to 1 compression was possible because we were talking static compression and the camshaft profile back then would bleed off a lot of pressure...and we had good racing gas and the connecting rods were huge, as were cranks and bearings..

 

[Kozy]

I thought NASCAR ran NASP engines though, so only ~1 BAR MAP?

I've only managed a quick scan read so far, but there's no mention of boost pressures in that article?

 

[jack action]

It would be interesting to see an example of these engines you refer to and know what kind of fuel it runs on.

Nonetheless, with computer-controlled ignition (+ knock sensors) and the possibility of switching camshaft at higher RPM, it seems conceivable. If you modify your «high-rpm» cam to have an extra long duration, your dynamic CR will be noticeably lowered ... but you won't notice it at idle - like in the old days - because you're using the «low-rpm» cam.

 

[Kozy]

Correctly assumed there Jack, these are Honda VTEC engines so the low and high cam thing is correct.

 

[Ranger Mike]

i concede that nascar and pro stock drag engines are normally aspirated..no turbos..

 

[S_Happens]

Like jack action mentioned, you can change your valve timing to alter the dynamic compression ratio. This doesn't have to be done with variable valve timing, but that can optimize for separate situations.

Water/methanol injection can also be used.

 

[Kozy]

You can do that, yes, however the standard cars still have that luxury and yet run 10:1 at 1BAR MAP, I am not sure they are mitigating the compression pressures at 14:1 and 2BAR MAP with cam timing alone?

 

[Pkruse]

I have little knowledge of racing engines. My experience is with practical working engines. It is common to reduce the pressure ratio when adding boost. The idea is to give the combustion chamber a little more volume. Without the boost, that would mean less pressure. But with boost, you keep the pressure about the same. That enables you to get more fuel and air into burn, while using the same fuel.

 

[Kozy]

Yes that's the normal method of building a boosted engine, so I'm wondering how some people expect to be able to reduce the combustion chamber volume AND increase the boost pressure, and yet make it run with destroying itself?

If we go from 85% VE to 127.5%, an increase of 50%, in order to keep the cylinder pressure the same as stock with 120% VE the CR should be reduced to around 8.5:1.

Going from an 11:1 CR at 85% VE to 13:1 and 127.5% VE is going to increase peak cylinder pressure by around 40-45% before it is even ignited. By my calculations, that's even more cylinder pressure than the standard engine would make with 30° ignition advance, purely from the compression stroke alone!

Can all this excess pressure really be mitigated purely with retarded intake cam timing? What other effects is this going to have on the motor? Is it really worth the effort over a lower compression engine?

 

[Kozy]

Further reading reveals that these engines are infact running on E85 or similar fuels which would probably explain it.

 

[Pkruse]

Yes, Kozy, alcohol burns in a manner similar to high octane gasoline. It has less energy content, but if you increase the combustion chamber pressures, the increase in efficiency can more than make up for the lower energy content of the fuel. But burning it without matching the engine to the fuel will result in a higher fuel burn rate.

 

[HowlerMonkey]

It's fuel choice that determines your success in this...and how much it costs to fuel it.

 

[Kozy]

I've been discussing a very similar subject elsewhere, and thought I'd post up the following which I have been working on.

I've been doing some research into engine thermodynamics and I've come up with some plots of what changes when you turbocharge a factory NA engine. Thought I'd post them here with my thoughts on it for discussion.

Anyone who's ever sat a thermodynamics class will be familar with a PV diagram for an otto cycle. If not, some links here and here

Here's one comparing two engines with B18C geometry, both 11:1 compression, 45° MBT ignition timing, but one force fed 0.5bar to 150% VE. All plots taken at 7200rpm and 13.5AFR.

Here is the torque output comparison for one cylinder over one engine cycle.

Clearly the boosted engine is producing a huge amount more cylinder pressure at peak (over 1300psi), which while it makes a huge amount of torque, 234lbft vs 156lbft, it would most likely destroy the engine pretty quckly. The NA engine peaks at just under 900psi, so we'll take this as the safe limit for cylinder pressure.

Below, I've reduced the ignition timing back to 28.5°, the peak cylinder pressure is back at 900psi, but importantly the average pressure has risen from 210psi to 266psi, resulting in 213lbft. This would probably run pretty well.

Next, we'll up the boost to 1bar, for 200% VE. This requires the ignition to be dropped back to 16° to keep the peak cylinder pressure under 900psi.

I'd imagine this would not run all that well, but I could be wrong. The hugely retarded timing is obvious just past peak as the pressure drops before the fuel burn really gets going. It makes 244lbft.

Now, let's leave that engine as it is, and take the other one up to 200% VE, but with a 9:1 compression:

Ignition set to 25.4°, torque now at 264lbft. Lots more area visible, and the torque trace shows a gain pretty much everywhere unlike the previous ones which were a drain on torque on the compression stroke. This to me seems a better design for this level of boost.