Compression ratio

[rev-erend]

Here's a question :

Why does raising the CR improve the power output on a petrol engine.

[HarryW]

more squish = bigger bang in my simple mind

[wheeljack]

It's not so much compression ratio as it's expansion ratio. The more energy you can extract out of the expanding gas, the more work you get. Obviously geomtric compression ratio and expansion ratio are the same (or nearly) in conventional reciprocating engines, so the 2 get interchanged.

[GreenV8S]

If you raise the temperature and pressure at ignition the combustion occurs quicker, which means that you get more slightly less pressure during the end of the compression stroke and slightly more pressure throughout the expansion stroke.

[350matt]

So how much are you pumping the SEAC up by then Rev?

Matt

[rev-erend]

350matt said:

So how much are you pumping the SEAC up by then Rev?

Matt

18 to one..

going for a 5 litre diesel like the griff 500

No seriously - 11:0

It's just someone asked me the question and I thought about it and just knew it probably increased the pressure before the bust but was not sure if that was the true explanation..

Looks like det could be my new friend .. luckily I'm going fully mapped

[350zwelgje]

And in the process the fuel economy goes up as well, as the engine (in theory) gets more fuel efficient, so less fuel for same power. So raising the compression could get you a more economic engine! Until you start to reach other restrictions/limitations enginewise and end up with needing 120 octane fuel etc......



Rob

[Fat Richie]

350zwelgje said:

And in the process the fuel economy goes up as well, as the engine (in theory) gets more fuel efficient, so less fuel for same power. So raising the compression could get you a more economic engine! Until you start to reach other restrictions/limitations enginewise and end up with needing 120 octane fuel etc......Rob

So, following on from that, based on using an engine in a road car using normal unleaded fuel what is the highest CR that you can run in a particular engine? - I appreciate that it will vary from engine to engine!

Anyone got any figures for, just to kick things off...

BMC A-series

BMC B-Series

Ford Crossflow

Ford Pinto

Ford Zetec

Rover V8

Vauxhall 16v T/C (forget what it's called!)

[GreenV8S]

Fat Richie said:

So, following on from that, based on using an engine in a road car using normal unleaded fuel what is the highest CR that you can run in a particular engine? - I appreciate that it will vary from engine to engine!

Anyone got any figures for, just to kick things off...

BMC A-series

BMC B-Series

Ford Crossflow

Ford Pinto

Ford Zetec

Rover V8

Vauxhall 16v T/C (forget what it's called!)

There are various ways of defining compression ratio. The nominal static compression ratio is the usual starting point but the actual pressure (i.e. charge density) is more important. This depends on the static compression ratio but also factors such as cam timing and induction losses. If you have an asthmatic engine with a wild cam you might have a really high static compression ratio but still not get higher pressure than another engine with much less static compression.

[HarryW]

the static compression is all but academic imho, however a dynamic compression of 8 to 9:1 is where you want to be for a NA engine, again imho.

[Mave]

Even without considering the effect of pressure on combustion speed, increasing the CR increases power. This is because cold gas takes less energy to compress than hot gas, and vice versa ie hot gas gives out more energy when you let it expand than cold.



So, use small amount of power to compress air, heat it up, get more power out than you used to compress it, which is the thermodynamic basis of most internal combustion engines. Unfortunately you need loads of power in the form of fuel to do the heating, which is why it doesn't give you perpetual motion