Compression Ratios and their effect on torque


kachok25
12-23-2005, 02:02 PM
I know that higher CR produce more torque than lower ones I have heard that a 10:1 CR makes 5% more torque than a 9:1 CR and consumes 20% less fuel. And I know that the 17:1 CR is the Ideal compression, but how much more torque can we get than the 10:1 before the law of deminishing returns kicks in? And why does the higher CR require a lower A/F ratio? It is kind of hard to compare since virtualy all engines runing a 17:1 are alcohol burning which has the unfair advantage of carrying extra oxygen in with it. Anybody understand what I am talking about? If so give me your imput.

curtis73
12-23-2005, 03:24 PM
I know that higher CR produce more torque than lower ones I have heard that a 10:1 CR makes 5% more torque than a 9:1 CR and consumes 20% less fuel.

Increasing compression has a range of most effectiveness that lies sorta between 8 and 12. Between about 8 and 12 compression, a one point increase raises torque about 5%. But the difference between, say, 12 and 13 the gains are less. Then between 13 and 14 a little less yet. As far as I know that only applies for gasoline.

Now I'm going to get on a bit of a soapbox so forgive me. Compression ratio DOES NOT necessarily change how much fuel is consumed by the engine. People confuse combustion efficiency (which is what raising the compression benefits) with fuel efficiency. They have NOTHING to do with each other.

Greater combustion efficiency from increasing compression only means that the work given into the fuel pays greater dividends by more BTUs released from the fuel. The net result is that the increase in BTUs available is greater than the extra pressure (and therefore work) that the engine has to squeeze on the compression stroke. That only means that the engine is getting more push on the pistons. It also has minimal but measureable effects on intake air dynamics, but not important as far as this discussion is concerned.

Fuel efficiency is strictly how much fuel the engine ingests over the course of a measured distance. Its still consuming the same amount of fuel (especially if we're dealing with a carburetor). Changing the compression ratio doesn't change how much fuel flows through the injector or carb's booster. Where some people see an increase in MPG is due to the engine running with more combustion efficiency, which makes better use of the fuel you give it, i.e. they have to use less foot to maintain the same speed which (depending on at least a dozen factors) may or may not reduce the amount of fuel the engine consumes. In my case I tend to really play around with the new power I'm given from a modification and my right foot spends a lot of time smashed to the floor so my MPGs go to hell in a handbasket. My Impala SS was reprogrammed with a more advanced spark and leaner fuel curve which should have saved me at least a few MPGs, but in truth I spend so much time putting a grin on my face that my in-town mileage went from 18 to 14 :)

If your car is already running above peak cruise efficiency, chances are a compression increase will actually lower your MPGs. If you're below your peak cruise's RPM, in many cases you can realize a bit better MPG.

Efficiency means two different things in this case. One sometimes has an effect on the other, but they are independently exclusive parameters.

Can you expand on the part of your post concerning the 17:1? Is that the A/F ratio or CR? For CR, the point of diminishing returns seems to come at around 12 or 13:1 for gas, 21 or 22:1 for diesel, fuel oil, jet fuel, and biodiesel.

kachok25
12-23-2005, 03:57 PM
Increasing compression has a range of most effectiveness that lies sorta between 8 and 12. Between about 8 and 12 compression, a one point increase raises torque about 5%. But the difference between, say, 12 and 13 the gains are less. Then between 13 and 14 a little less yet. As far as I know that only applies for gasoline.

Now I'm going to get on a bit of a soapbox so forgive me. Compression ratio DOES NOT necessarily change how much fuel is consumed by the engine. People confuse combustion efficiency (which is what raising the compression benefits) with fuel efficiency. They have NOTHING to do with each other.

Greater combustion efficiency from increasing compression only means that the work given into the fuel pays greater dividends by more BTUs released from the fuel. The net result is that the increase in BTUs available is greater than the extra pressure (and therefore work) that the engine has to squeeze on the compression stroke. That only means that the engine is getting more push on the pistons. It also has minimal but measureable effects on intake air dynamics, but not important as far as this discussion is concerned.

Fuel efficiency is strictly how much fuel the engine ingests over the course of a measured distance. Its still consuming the same amount of fuel (especially if we're dealing with a carburetor). Changing the compression ratio doesn't change how much fuel flows through the injector or carb's booster. Where some people see an increase in MPG is due to the engine running with more combustion efficiency, which makes better use of the fuel you give it, i.e. they have to use less foot to maintain the same speed which (depending on at least a dozen factors) may or may not reduce the amount of fuel the engine consumes. In my case I tend to really play around with the new power I'm given from a modification and my right foot spends a lot of time smashed to the floor so my MPGs go to hell in a handbasket. My Impala SS was reprogrammed with a more advanced spark and leaner fuel curve which should have saved me at least a few MPGs, but in truth I spend so much time putting a grin on my face that my in-town mileage went from 18 to 14 :)

If your car is already running above peak cruise efficiency, chances are a compression increase will actually lower your MPGs. If you're below your peak cruise's RPM, in many cases you can realize a bit better MPG.

Efficiency means two different things in this case. One sometimes has an effect on the other, but they are independently exclusive parameters.

Can you expand on the part of your post concerning the 17:1? Is that the A/F ratio or CR? For CR, the point of diminishing returns seems to come at around 12 or 13:1 for gas, 21 or 22:1 for diesel, fuel oil, jet fuel, and biodiesel.

OK here is the best that I understand this tech artical from a GM engineer. They built and tested engines of different CRs and they progresivaly got more and more torque intill they reached 17:1 CR after that the extra compression no longer helped it hurt the torque. The reason Diesels sometimes run 22:1 is because they are compression ignition and they have to have high compression. But the better diesels do use a 17:1 like the Cummings turbo.
Yes increasing the CR does not make the ideal 13:1 air to fuel ratio any different, but at lower compressions they have to use higher A/F ratios, just look at a A/F map for a turbo engine, they run very high before the turbo spools up because of their low compression, this is for consistency in ignition, on a higher CR engine the fuel molicules are bunched up more tightly so ignition is more consistant without having to add more fuel, at leased that is the way I understand it, so while a high CR does not change the laws of physics by changing the way air and fuel react it does practicaly change how much fuel you need for pracice ignition, again I am not engineer I have never tested this in a lab but that is the way I understand it, and that does make sence when you think about it. Why else would a 5.7L high CR LS1 get better fuel efecency that a 2.0L 4g63 in a lighter car?

beef_bourito
12-23-2005, 04:19 PM
I'm not so sure about the diesels being 17:1 cr. the way i understand it, the hotter the air in the combustion chamber, the faster the fuel reacts with the air (basic chemistry, as heat goes up, so does the rate of reaction). so when you increase the compression and/or boost going into a diesel engine, the diesel reacts more quickly so you can burn more fuel during the power stroke. the problem with diesels when they add more fuel is that they can't always burn it all even though there is enough air in the cylender because diesel burns slowly.

kachok25
12-23-2005, 05:07 PM
I'm not so sure about the diesels being 17:1 cr. the way i understand it, the hotter the air in the combustion chamber, the faster the fuel reacts with the air (basic chemistry, as heat goes up, so does the rate of reaction). so when you increase the compression and/or boost going into a diesel engine, the diesel reacts more quickly so you can burn more fuel during the power stroke. the problem with diesels when they add more fuel is that they can't always burn it all even though there is enough air in the cylender because diesel burns slowly.

The cummings turbo is 17:1 I looked it up, the reason it can run well at 17:1 is becasue it is turbocharged, hence it crams more air into the cylinder making higher pressure and more heat. The NA 6.2 and 6.5 I used to work on all the time are 22:1 if memory serves, but those things were crap in my book. There is good reason why GM replaced them with the duramax, which I bet if you look up you will find it is near 17:1 too, not for sure but I would almost bet money on it, since it was GM that did the reserch that I am talking about.

beef_bourito
12-23-2005, 05:42 PM
I'm not saying that there aren't any 17:1 diesels, I'm just saying I don't think that that cr is the best.

kachok25
12-23-2005, 06:51 PM
I did not start this thread for disele engines in all actuality, but if anyone here knows of a better one than the 5.9 Cummings turbo let me know LOL I am actualy talking about gasoline/alcohol engines to be honest. But just for the sake of entertainment I'll try to look up the powerstroke and duramax CR too. BRB

kachok25
12-23-2005, 06:59 PM
pre 2006 duramax 17.5:1 CR
2006 duramax 16.8:1 CR
New powerstroke 17.5:1 CR
Do I still have any unbelievers in the crowd
The 17:1 CR is the optimum for the internal combustion engine +or- a point of compression depending on other factors. :smokin:

curtis73
12-23-2005, 07:18 PM
Nope, I believe you. :) Those diesels have lower compression for the turbo's sake. Many non-turbo diesels have higher CRs. But that shouldn't be mistaken for an optimum CR for diesel, its just the CR that will make it operate without blowing the head bolts off :)

But, back to the topic at hand. I'm not sure why there would be a benefit to a richer mixture for less compression. I understand your demonstration about them being packed closer together with more CR, but the fact is that there are still only 14.67 liters of air required to combust one liter of gasoline. I wonder if the turbo crew makes wetter curves while not under boost to help smooth out idle or something. Often times they have to use such large injectors to supply enough fuel under boost that they're too large to idle at the proper delivery.

I've also heard that mixtures are most of the time purposely tuned richer under boost to help cool the combustion and give a safe barrier against detonation.

kachok25
12-23-2005, 07:50 PM
Nope, I believe you. :) Those diesels have lower compression for the turbo's sake. Many non-turbo diesels have higher CRs. But that shouldn't be mistaken for an optimum CR for diesel, its just the CR that will make it operate without blowing the head bolts off :)

But, back to the topic at hand. I'm not sure why there would be a benefit to a richer mixture for less compression. I understand your demonstration about them being packed closer together with more CR, but the fact is that there are still only 14.67 liters of air required to combust one liter of gasoline. I wonder if the turbo crew makes wetter curves while not under boost to help smooth out idle or something. Often times they have to use such large injectors to supply enough fuel under boost that they're too large to idle at the proper delivery.

I've also heard that mixtures are most of the time purposely tuned richer under boost to help cool the combustion and give a safe barrier against detonation.

Yes optimum fuel mixtures are the same no matter what the pressure, but that is the difference between statisic and actual practicall aplacation, I'll look for some other good examples that are non turbo. I read an intresting figure once back in the early 90s that although 13-14 to one was the perfect AF ratio most passenger cars ran around a 9:1 AF ratio. I don't know if that is completly true but I know that a good racing engine can run much leaner than that. I'll do some more reserch and get back with you.

Altimas
12-26-2005, 10:06 PM
No passenger car runs around 9:1 AFR under normal conditions that is functioning properly.
That would obviously result is extremely poor gas mileage and would make the engine make less power and black smoke emit from the tailpipe.
Under part throttle conditions cars with three way cars should go slightly leaner than 14.7:1 and slightly richer so as to have the cat work properly to reduce all the measured (bad) emissions.
Under full throttle they might go as rich as maybe 12:1 for turbo cars but as you said around 13:1 is about the perfect AFR for most cars.

Tuned rotaries, on the other hand, may go as low as ~9:1 AFR for highly boosted applications since even 1 detonation on a rotary can kill the apex seals.

Floyd_ODB
12-26-2005, 11:01 PM
I think you might be thinking that alcohol engines run 9:1 or " richer* than that"
most alky motors I've seen were between 12 and 14:1 compression.
I think gas ignites(knocks) to easily under these CR's but I dont know.
are you wanting a 17:1 alky engine? for racing?

kachok25
12-27-2005, 12:44 AM
No not really I am just studying my stuff for future refrence. I am actualy planning to build a forced induction 7M-GTE or VG30dett with suplamental alcohol or toluine injection. Low end torque with racecar power with the turbo spooled up, running on pump gas under normal conditions, it will be the the perfect street engine, with 10 second 1/4 mile power just an inch of throtle away.

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