rover v8 compression questions?

[tomtomtom]

why did land rover decide to make different compression ratios on there engine whats the benefit of a low compression engine? and what makes an engine lower or higher compression cheers

 

[Satancom]

The low compresion Rover V8 has a very pronounced dish in the top of the piston where as the high compression has a shollower dish giving a smaller space to compress the fuel/air mixture. The lower compression engines have less power but will be less strained and will run on lower octane fuel. Range rovers were usually high compression 9:1 (ish), the Land rover stage 1 was 8:1 or so. Some of the earlier V8's were nearer 10:1 and woulad need to run on super unleaded or with an octane booster to prevent pinking!

Or something along those lines

 

[tomtomtom]

so you could simply put some higher com pistons in there? also what was the point in making the engine lower com, more torque?

 

[jozg44]

also what was the point in making the engine lower com, more torque?

As Satancom said, it's to allow the engine to run on low-octane and generally poor quality fuel. With higher CRs you run the risk of two problems: 'knock' (essentially uneven and erratic combustion of the fuel/air mix after the spark plug has fired, caused by a combination of high compression and poor fuel) and 'pre ignition' (where the mixture ignites before the spark plug fires and when the piston is still rising. In bad cases this can destroy an engine in under a minute).

Lowering the compression ratio reduces the chances of both of these but there are other aspects at play (tuning, mixture, combustion chamber design). In either case lowering the CR reduces engine efficiency, power and torque. But for a Land Rover the ability to use rubbish fuel under heavy load is more important than performance.

Of course all this can be avoided by using diesel engines which, because there is no fuel in the cylinder during compression, can have a hugely higher CR which give you much better efficiency.

Jack

 

[Llanigraham]

Of course all this can be avoided by using diesel engines which, because there is no fuel in the cylinder during compression, can have a hugely higher CR which give you much better efficiency.

So what goes "BANG" then?
The diesel is a "compression ignition" engine.

 

[jozg44]

So what goes "BANG" then?
The diesel is a "compression ignition" engine.

The fuel is injected after compression. If you're being picky fuel injection takes place a few degrees before the piston has finished rising on the compression stroke to take into account the time delay of the fuel pulse to travel from the injector pump to the injector and the (very, very small) delay between the fuel being injected and it igniting.

It's 'compression ignition' because the heat for ignition comes entirely from the compressed air- no external source required, leaving aside glow plugs and the like to get things started.

Jack

 

[zollaf]

a lower compression ratio puts the torque lower down the rev range. a 7:1 comp 2 1/4 can tickle along on tickover all day quite happily.

 

[zedcars]

Llanigraham
Three phases of combustion in a CI engine.
Air is compressed to about 450 dgf at about 30 to 15 degrees btdc --Phase one (depend upon engine design)

Fuel is injected and the latent heat is absorbed by the fuel to a point of combustion this depending upon design takes as much as 5 to 15 degrees of crank rotation to create actual combustion in the cylinder---Phase two

(Remember there is an instant drop in cylinder pressure, Boyles Law which take a few degrees of crank rotation to return the pressure & heat necessary to commence combustion )

This is also known as the delay period and gives the diesel engine its synonymous "Knock"

Phase Three
Heavy downward constant pressure is exerted upon the piston providing the cylinder expansion giving the CI or diesel engine its inherent torque producing power.

An indicator diagram will show the distinct pressure differences when comparing a spark ignition and CI engine.
Dennis
zedcars

 

[phoenixdave]

a lower compression ratio puts the torque lower down the rev range. a 7:1 comp 2 1/4 can tickle along on tickover all day quite happily.

True, and for similar reasons a low compression engine will tick over smoothly at lower rpm. I have also noticed that on lpg fuel my 8:1 2.25 will pull smoothly and tick over at much lower rpm than it will on petrol.

David

 

[zollaf]

less energy required to compress the air, thus more energy to the flywheel. and indeed, i can get my 2 1/4 in my trialler which is 7.5 :1 or thereabouts ticking over at about 300 rpm.

 

[DarrenH]

science over load !

original question, compression ratio is size of cylinder/gasket space/piston dish/combustion chamber in head...when the piston is at the bottom of the cylinder. divided by the size of those areas at top dead centre.

so in a 9:1 engine, the space is 9 times bigger at bottom of the stroke, to the top of the stroke.

what makes it higher or lower compression, you reduce the size of the combustion chamber in the head (or not have one at all like some diesels) or make the piston bigger with the chamber size unchanged.

to some degree its by the by as the cylinders only every fill to about 70-80% capacity (normally aspirated engines) and at a narrow point in the rev range, so the actual compression ratio (called dynamic compression) is allot less

 

[phoenixdave]

science over load !

original question, compression ratio is size of cylinder/gasket space/piston dish/combustion chamber in head...when the piston is at the bottom of the cylinder. divided by the size of those areas at top dead centre.

so in a 9:1 engine, the space is 9 times bigger at bottom of the stroke, to the top of the stroke.

what makes it higher or lower compression, you reduce the size of the combustion chamber in the head (or not have one at all like some diesels) or make the piston bigger with the chamber size unchanged.

to some degree its by the by as the cylinders only every fill to about 70-80% capacity (normally aspirated engines) and at a narrow point in the rev range, so the actual compression ratio (called dynamic compression) is allot less

All nicely explained, Darren. I would beg to differ with your last sentence though. The compression ratio will always be what it is, but because the cylinders are never fill more than 70-80%, the compression pressure will be less than you might think, giving a lower effective compression ratio. Fit a turbo/supercharger though and you are entering a whole new ball game!!

David