Compression ratio question

[dicer]

Does anyone know what the highest compression ratio that has been tested is? And who or where? Recip or free piston?
And test results in terms of power, efficiency etc.

 

[FOURe]

Hi there:

In my opinion, regular compression ratio is ~ 15 to 20 [/].

I have no idea what the achievable highest compression ratio is.

The higher the compression ratio, the more difficult it gets to make it work ...

In general, there is always some kind of an optimum combination of compression ratio, equipment design, maintenance and associated cost ...

Here are a few general compression plots:

​

Thnaks,

Gordan

http://www.engineering-4e.com

 

[BrianPetersen]

Since you didn't specify the type of engine or fuel ...

The earlier indirect-injection VW diesels were above 22:1.

High specific power output was not one of the features of those engines.

 

[FOURe]

Here are a few typical ideal Otto and Diesel cycle plots:

​

http://www.engineering-4e.com

 

[Tmoose]

At some point in the conversation the difference between mechanical ( full stroke ) and "trapped" compression ratios may come up. The octane requirements of a 4 stroke gasoline engine with 13-1 compression ratio depend a lot on how late (deep into the compression stroke) the intake valve closes.

IN the 70s or 80s during another energy crisis Bruce Crower (Cams) played with peculiar intake timing and a high mechanical compression ratio to create unequal compression and expansion ( more complete expansion) ratios.

For performance piston port 2 strokes of the 70s the exhaust port closes around mid-stroke. Thus the oily low octane mixture inhaled by a Montesa 125 with over 13-1 brochure compression is really not much more tightly squeezed than a Yamaha with 7.X-1

 

[BrianGar]

Compression ratio depends on a whole load of stuff from cam spec(Dynamic c/r)to the type ignition system, fuel quality, knock control, etc... .

 

[SMOKEY44211]

Mathematically compression and expansion ratio are interchangeable. As TMOOSE alluded to mechanically the values can be altered by changes in valve timing. Example 10:1 compression/expansion ratio engine will yeild~150 psi cranking pressure readings if intake valve is closed at 180 deg. atdc. If we take the same engine and close the intake at 270deg. atdc guage reading will be~75 psi. Assuming exhaust valve opening event remains the same in both cases the expansion ratio will remain the same. In a gasoline fueled engine it is the pre combustion pressure and temperature for a given octane value that determines what the highest value mechanical compression ratio can be. According to an SAE paper published by a GM engineer 17:1 compression ratio is the high limit for a gasoline fueled engine. Any gains in power and improved BSFC are really the result of raising the expansion ratio. In Haywood's book he touches on the subject he calls hyper expansion reporting what I just stated.---------Phil

 

[pecki]

Hi guys,

I'm a french so excuse me if i speak a bad english.

When you upgrade your compression ratio, you have more power.

The principale effect caused by the increase of Cr are improve your cycle yield (thermodynamic effect) and augment your pollution (combustion effect).

The main problem of a Cr too high, is the apparition of detonation (like knock in the cylinder). But, in race engine the detonation is counter detonation by a different Air Fuel ratio (AFR), they augment fuel on AFR and less ignition advance.

It's difficult to give you the highest Cr has been tested because of this effects but in an ignition engine, like SMOKEY44211 say, it's near of 17:1 with gasoline. There's an other gasoline engine without ignition (it work like a diesel, it's the HCCI process) and the Cr is more about 20:1 to 22:1

Bests regards, pecki

 

[dcasto]

Peak thermo efficency is around 13 to 1. Just over a typical maximum spark ignition to be be used and just under a ratio needed to get enough POWER from a compression ignition.

 

[evelrod]

With my race engines in the 15 to 16:1 CR range, I don't find it improbable that the pro NASCAR engines would be even higher. My engine operates quite well on VP 110 and the pro types are using Sunoco 116 (at least).

It also does not surprise me that even little engines like the Prius or several of the other VVT engines are operating of pump gas with CR's of 13+:1 !

Rod

 

[dicer]

The question is open to all forms of piston type engines all fuel types etc.

All the listed ratio's so far are pretty standard stuff, no one knows of any special tests in this area?

 

[YvesLLewelyn]

I doubt if really high CRs have been seriously tested. With unrestricted manifold pressure detonation will always occur. I have run a petrol engine at an 18:1 CR. The engine was intended for Atkinson Cycle/limited manifold pressure use but I ran it unrestricted just to see what would happen. Needless to say it detonated. Some of the effects were interesting - when detonating the smell of the exhaust was like teargas - presumably this was due to the formation of peroxides and aldehydes etc.
Also, when detonating there was black smoke coming from the exhaust - my simplistic explanation was that the "bang" of the detonation actually put the fire out resulting in unburnt or only partly burnt fuel. This is in the same sense that explosives have been used in the past to put out oil well fires etc.
The highest CR I have seen on an unrestricted engine is about 30:1 used on a model plane "diesel" engine.

 

[izzmus]

I forget where I read it (possibly intentionally) but there was an engine builder experimenting with combustion chamber shapes who had pump-fuel CRs in the 20-23:1 range.

 

[j2bprometheus]

Dicer-

I vaguely remember a NASA paper about a free piston engine with compression ratios of about 100. I think that this was analysis only, but I am not certain.


It was completely impractical.

As compression ratio goes up, peak cylinder pressure goes up for the same power level. All real engines have peak cylinder pressure limits. Example: I recall GM bragging 5-7 years ago that their new in line six gasoline engine could take "high" peak cylinder pressures. They thought that 1000 psia was high. Diesels are typically limited to about 3000 psia.

Also, as peak cylinder pressures go up, temperatures go up, so NOX goes up.


j2bprometheus

 

[patprimmer]

Also the weight of reciprocating parts tends to go up.

Regards
Pat
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[dicer]

izzmus, If memory serves, I think Detroit multifuel engines were in the 25:1 area, it would need to be to burn high octane gasoline as compression ignition.

I ran into information some time ago, that had mentioned 300:1 and this is why I asked the question, I'm thinking it was free piston arrangement.

 

[tbuelna]

Production small bore N/A IDI diesels require geometric CR's around 22 or 23:1 simply to allow them to be started. I personally worked on a 2-stroke diesel with a 12:1 CR and using over 100psig of manifold pressure, which would be equivalent to something like an 80:1 CR(?). I'm sure there have been more extreme examples.

What's really more important than simple geometric CR's are cycle pressure ratios and temperatures. These really determine how much work the engine system has extracted from the thermal cycle. Large turbine engines are now getting close to PR's of 45 or more.

With regards to efficiency and emissions in a highly boosted DI diesel, a geometric CR around 12:1 would be ideal, but it would be very difficult to start. Most medium sized diesels use something around 17:1 with lots of boost and charge cooling, which results in good efficiency and low emissions. Very large bore loop-scavenged 2-strokes (like ship or stationary engines) with auxiliary supercharger drives and mega turbocharging can get away with geometric CR's around 12 or 13:1. The best compromise for efficiency in recip engines appears to be to limit the compression and expansion work done in the cylinder, while also doing as much as possible with external turbo machinery. Thus, you will not likely find too many examples of recip engines with extreme CR's.

Regards,
Terry

 

[outlaw695]

The highest operating comp. ration i know of is 52:1, running nitromethane, with a further 45psi boost. obviously a drag engine, small bore controls detonation somewhat. power is over 730HP per 1000cc. Large camshaft duration also bleeds some cylinder pressure but more to do with the stability of the fuel in these conditions

 

[PEW]

Not strictly on topic, but added out of general interest: The present land speed record holding diesel engined car has a CR of just 10 to 1.

 

[patprimmer]

Yes, but what what boost level and what supercharging methods and what starting method.

Regards
Pat
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