COMPRESSION VS INTERCOOLING VS ALCOHOL/WATER INJECTION

[Wilkdan]

I have had this debate ongoing with other specilist mechanic and it is quite a good subject to get different views and experiences;

If you had an engine with a 9;1 compression ratio and you want to run a high boost which option would you go for or a bit of both;

My view is to try and keep compression up and run a large intercooler (where possible) and may alchol injection to prevent detonation and a compression ratio change as a last option!

What's your opinion

 

[Lou Scannon]

Way too many variables to have a hard & fast answer.
How do the priorities stack up? -
maximum power (drives you to lower compression for greater knock-limited MEP tolerance)
cost of operation (drives you to raise compression for greater thermal efficiency)
initial cost
convenience (two fluids to replenish)
reliability (alcohol/water setup is another failure point)
engine management system limitations (air flow/MAP ceiling, fuel delivery ceiling, spark voltage ceiling)
etc.

 

[Warmington]

Whats the target goal in terms of bhp/litre and at what revs?

Intercooling and water injection maximised to the point of detonation at your peak torque, then if more power is needed, decresae Rv and up the boost!

 

[WickedWeasel]

My friends own a high performace Corvette shop and specialize in FI and NA.

I help them out with sales and support and really know the NA side since I road race, but my knowledge of FI is still limited.

the knowledge I do have says that water methanol injection is the way to go with a higher boost engines. The benefits of cooling the air, being able to run on pump gas without detonation and being able to add more boost are probably the best reason why.

Now if you want more techincal answers I will have to go ask my friends who are at PRI right now, but will be back Monday.

 

[Wilkdan]

I know there is loads of variables;

i specilise in BMW forced induction and tuning and i find that anything of high boost application (anything over 1.5 bar) is something for concern; I tried to stay with pump gas for road application otherwise its more hassle then its worth;
I like to run around the 1.5 bar with my BMW 328; I haven't lowered my compression ratio from 9;1 but run a large intercooler instead; i did suffer from knock under high boost but solved this by running a piggy back ecu which controlled the boost with the knock;

I am thinking about a new idea which is what formula 1 used and cooling the fuel down; this is something i am tempted to develop any experience with this??

 

[Lou Scannon]

Colder fuel is of limited help, since it is a relatively small mass fraction of the mixture. More liquid evaporating in the cylinder (whether water or fuel) is a much more powerful knob to use due to the heat of vaporization.
There is a recent application by GM or Ford in a supercharged pickup that uses the A/C system to cool the charge under heavy boost.

 

[Wilkdan]

So if i am cooling the air and cooling the fuel then i should be able to run high boost without detonation;

Would you all think that the power lost by running an a/c system to cool the AIR would be more then the denser air would produce in power;
I was thinking about doing this by finding a small air con system and running the aircon pump geared up with a power steering electric motor off a MINI cooper; they dont drain to much amps so the alternator won't draw so much power off the engine???

 

[Lou Scannon]

in principle driving the a/c compressor directly is more efficient since you avoid any electrical losses. That assumes the compressor is being driven at an efficient rpm.
I don't know much about the OEM system I mentioned, it may be that you pre-cool a heat sink prior to using the feature.

 

[WickedWeasel]

I believe that many cars turn off the air conditioner at wide open throttle (at least corvette).

you could change the tune to leave the air on, but redirect it to help cool the air. It may work, but the money and time spent would probably cost more than meth injection. Also you would lose power at WOT where you want it most.

ECSRacing

http://www.ecsracing.com

 

[Wilkdan]

If i run the A/c pump directly then it will only run at the engine speed/ if i run the pump ac compressor geared up off the motor then i can have it run at optimum speed anf with a high efficient altnernator;

I was going to run a matrix in the inlet which cools the chrage before entering the inlet manifold or maybe try to cool the plenum down

 

[Lou Scannon]

I found this site discussing Ford's "Supercooling" technology:

http://www.motortrend.com/features/performance/112_0309_2004_ram_srt10_lightning_svt/engine.html

They claim 50 extra HP for up to 45 seconds; that's a pretty worthwhile benefit.

 

[patprimmer]

You could run the a/c of an electric motor (or a small seperate petrol motor) the use a larger alternator if required, but disable the alternator at WOT by either switching off the current from the regulator or by an A/C type clutch on the pulley.

Hardly worth the trouble though.

I see a progressive system like this.

Increase boost to increase power.

Use best fuel reasonably available and practical.

If you want more boost, add intercooler.

More still add richer mixture or add water/alcohol injection.

More still, reduce compression but don't add WI, but still add intercooler.

More still, as above but slso with WI.

Maximum boost, use exotic fuel and intercooler and reduced compression and maybe two stage supercharging.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mattsooty]

Wilkdan,

My opinion, for what it’s worth….

Don’t waste your time with AC compressors and fuel coolers.

First thing to consider is a correctly matched turbocharger with high efficiency. This will keep compressor outlet temps as low as possible and make your task easier.

Any intercooler is going to act as a restriction in the boost circuit. It will also hurt driveability by introducing extra lag into the system due to its volume.

Keeping a high compression ratio is going to help fuel economy – which is not mentioned in the OP.

In terms of water injection (forget the alcohol, if you have some put it in with the fuel) this is going to give you some large benefits, up to a point. Then it becomes self-defeating as the H2O displaces the air from the cylinder and dramatically slows the burn time.

I would approach the problem by using said turbo with a small but low restriction intercooler sized to reduce the temps enough that knock is not an issue at part load. Then at full load and at large throttle transients I would use my water injection to suppress knock. Once you start reaching the limit of the WI effectiveness and you cannot retard the spark anymore due to component protection I would then reduce the CR stepwise until your given fuel is deemed safe.

MS

 

[Wilkdan]

matty,

i think you misunderstood,

i am just brain storming different ideas!!!

thanks for the help thought!!

 

[patprimmer]

One very important reason to put some alcohol in the water in the water injection system is to suppress the growth of algae.

If allowed to grow, algae can block water injection nozzles, which can suddenly cause catastrophic detonation at high speed maximum load. This can result in very extensive engine damage. Don't ask me how I know this.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Milanoguy2]

Wikidan

F1 teams used to cool their fuel to increase their fuel carry capacity, it was a fuel management strategy and had nothing to do with avoiding detonation.

In fact most teams used a heater to warm up the cooled fuel so it would atomize properly.

Static compression ratio in a turbo car is a compromise. Low static compression ratio leads to various bad thing like poor throttle response, and very low power off boost.

Very few turbo engines go below 7 to 1 CR. The Honda Turbo F1 motor of the early eighties had a CR of 7.4 in the 4 bar era raising to 9 CR in the 2 bar era.

If you like 9 to 1 stick with it.

Have you read Maximum Boost by Corky bell? If not you should.

Bye

 

[BrianPetersen]

One thing I've learned lately is that you need to pay attention to the shape of the combustion chamber and the flow patterns within it, also. This is NOT a simple task. Since you seem to be talking late model BMW, those have an ordinary pentroof 4 valve head with the sparkplug in the center. Anything you can do to get faster combustion will help. Anything you can do to focus the combustion chamber towards the exhaust-valve side of the chamber should help. You want to burn the mixture before it detonates and you want to burn the hottest (and most detonation-prone) parts of the mixture first.

On a forced-induction engine, you normally want mild cam timing with not much overlap. Use that to reduce the depth of the valve pockets in the pistons. Use THAT to put more of the chamber (1) closer to the spark plug (2) closer to the exhaust valves, or (3) arranged to give more squish turbulence.

How much money do you have for R&D. How much money do you have for custom pistons.

Read this (and there are a lot of other interesting articles on this same web site if you do some poking around):

http://www.gofastnews.com/board/engine-technology/33-cylinder-turbulence-burn-rate.html

 

[Wilkdan]

Thanks lads for the replys;

I have read maximum boost and currently reading the superchrger one by Bell;