Someone please explain engine compression ratios to me

[LTC8K6]

Data for those of us not in the US?

3.7L 305hp 280tq 31mpg Port injected
3.6L 304hp 273tq 29mpg Direct injected

 

[Fenixgoon]

3.7L 305hp 280tq 31mpg Port injected
3.6L 304hp 273tq 29mpg Direct injected

part of that is due to transmission gearing and vehicle weight though.

the GM's 3.6L is featured on the Camaro, CTS, and several other vehicles that are significantly heavier than the Mustang w/ the 3.7L

 

[lavaheadache]

Why the hell is this thread turning into a Ford vs Chevy thread? Come on people! besides being off topic, both suck!

It's like asking :

What tastes better? Cat shit or dog shit?

 

[LTC8K6]

part of that is due to transmission gearing and vehicle weight though.

the GM's 3.6L is featured on the Camaro, CTS, and several other vehicles that are significantly heavier than the Mustang w/ the 3.7L

Not HP or TQ, and weight won't effect hwy mpg much. The 3.6DI should handily trump the conventional 3.7L, but it actually loses in all three.

I suspect Ford has under rated the 3.7L in the Mustang too.

 

[yhelothar]

Why the hell is this thread turning into a Ford vs Chevy thread? Come on people! besides being off topic, both suck!

It's like asking :

What tastes better? Cat shit or dog shit?

probably cat shit... dogs eat that like candy. :awe:

 

[XZeroII]

Why the hell is this thread turning into a Ford vs Chevy thread? Come on people! besides being off topic, both suck!

It's like asking :

What tastes better? Cat shit or dog shit?

Cats bury it. This is a huge plus.

 

[yhelothar]

OK I think I get the turbo thing now. Since with forced induction, a lot more air needs to come in, you need more space to accomodate this air, thus meaning a lower compression ratio. The pressure when the piston is at its highest position will still be higher with an FI car/low compression ratio compared to an NA/high compression ratio.
Pre-ignition is produced by too high of a pressure/temperature of the fuel/air mixture.
So what are the ways of creating a higher compression ratio? It'd be to lower the temperature of the air right? Are there any other ways?

 

[Eli]

OK I think I get the turbo thing now. Since with forced induction, a lot more air needs to come in, you need more space to accomodate this air, thus meaning a lower compression ratio. The pressure when the piston is at its highest position will still be higher with an FI car/low compression ratio compared to an NA/high compression ratio.
Pre-ignition is produced by too high of a pressure/temperature of the fuel/air mixture.
So what are the ways of creating a higher compression ratio? It'd be to lower the temperature of the air right? Are there any other ways?

Combustion chamber design plays a huge role in it too. That's why one 10.5:1 CR engine can use regular while the next must use premium.

The easiest way to be able to run a higher compression ratio is to use a fuel with a higher octane.

For example, ethanol has an octane rating of 113. Because of this, alcohol fueled engines can run upwards of a 15:1 CR. This higher compression ratio also enables them to extract more energy out of the alcohol, which actually contains physically less energy than gasoline. The higher CR effectively increases the efficiency of the engine.

 

[yhelothar]

What makes diesel engines so slow then?

 

[Eli]

What makes diesel engines so slow then?

What do you mean so slow? As in maximum RPM, or how fast they move a car?

Diesel engines aren't "slow". They produce insane amounts of torque, which will give you that seat of your pants feel.

They don't rev very high due to their inherent design, so they typically produce less power than a comparable gasoline engine.

For example, the 6.7L PowerStroke in the 2011 F250 produces only 390HP, but 735ftlbs of torque, while the 6.2L gasoline engine produces 385HP but only 405ftlbs of torque.

I'm sure this isn't the best example, but it gives you the general idea. The higher compression ratio of the diesel engine is one of the reasons they get better fuel economy than their gasoline counterparts. They're more efficient.

The other reason is because diesel fuel contains more energy than gasoline.

 

[Stefan Payne]

Compression = power. The higher the compression ratio, the more power produced.

No, not really.
The compression ratio hasn't that much to do with the poweroutput but rather the efficiency of the engine.
ie the higher the compression ration = the more efficient the engine is.
But there are limits to that, so

The compression ratio must be lower to allow for the increased volume and resultant higher compression ratio. They both pack more air into the cylinder.

The compression ratio has to be lower so that the gas can be ignitet correctly.
If it's not it might ignite on it's own, without a spark from the spark plug, wich results in ringing.

What makes diesel engines so slow then?

That's due to the fuel, it limits the Diesel engine to about 5000rpm.
Basically the gas doesn't burn fast enough to let it rev higher, so that's the main reason for this.

I'm sure this isn't the best example, but it gives you the general idea. The higher compression ratio of the diesel engine is one of the reasons they get better fuel economy than their gasoline counterparts. They're more efficient.

Not really.

Keep in mind that a litre of Diesel has a lot more energy than a litre of gasoline fuel.
As far as i know it's about 20-25% more energy per litre.

 

[AMCRambler]

Compression = power. The higher the compression ratio, the more power produced. Lots of other things factor in, so this a general statement.

For a turbocharged or supercharged engine, the compression ratio must be lower. A turbo (supercharger too) pumps more air into the engine. The compression ratio must be lower to allow for the increased volume and resultant higher compression ratio. They both pack more air into the cylinder.

Boomerang nailed it.

 

[Eli]

Not really.

Keep in mind that a litre of Diesel has a lot more energy than a litre of gasoline fuel.
As far as i know it's about 20-25% more energy per litre.

It is most certainly both. I said that it's the increased efficiency combined with the higher energy per unit that enables diesels to get better fuel economy.

Also, it's about a 10% difference; A gallon of gasoline contains about 125,000BTU. A gallon of diesel contains about 138,000BTU.