Basic (stupid) questions about gas and diesel engines.
- Thread starter CobyRupert
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dodge man
Super Star Member
Just to answer one of your questions. A gas engine and compression. I get what you are asking, the cylinder pulls a vacuum, does that lower the compression ratio. Probably in theory yes but in the real world not by much.
I remember several years ago I read a story where they rebuilt an engine to exact factory specs. In this case I think it was a 440 Magnum. I can’t remember what the compression ratio was, say 10.5 to 1. The actual compression ratio was less than that. As I recall the engine made close to or right at the actual horsepower that was advertised on the dyno.
I remember several years ago I read a story where they rebuilt an engine to exact factory specs. In this case I think it was a 440 Magnum. I can’t remember what the compression ratio was, say 10.5 to 1. The actual compression ratio was less than that. As I recall the engine made close to or right at the actual horsepower that was advertised on the dyno.
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dodge man
Super Star Member
Another point on gas engines. Modern vehicles run pretty lean going down the road, this is for fuel mileage and emissions reasons. At full throttle they actually run slightly on the rich side. I’ve got a newer Challenger and had it on the dyno and they put a probe in the exhaust to read the air fuel ratio. On a modern car that ratio stays incredibly flat due to computer controls. On a carbureted engine that ratio is all over the place in comparison.
LouNY
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To the best of my knowledge most diesel common rail fuel injection systems do have an excess fuel return;
an example is a around 6:33 in this video.
Also all the common rail systems I have had anything to do with also had a return from the lift pump back to the tank.
When the filters are replaced part of the bleeding procedure is to cycle the key on a few times for the lift pump to pressurize the fuel feed system and purge the air out of the loop and back into the fuel tank.
MossRoad
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Hay Dude
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in regards to common rail Diesel engines, do NOT fill your fuel filters with diesel fuel from a tank before you screw them on, unless you have a really good filtration system.
There’s too much chance that unfiltered fuel in the filter will get to the common rail injectors and cause major problems.
The tolerances in newer common rail injectors is unbelievable. I had an injector tech tell me if you took a sharpie and put a mark on an injector plunger with it, it wouldn’t fit back in the injector.
There’s too much chance that unfiltered fuel in the filter will get to the common rail injectors and cause major problems.
The tolerances in newer common rail injectors is unbelievable. I had an injector tech tell me if you took a sharpie and put a mark on an injector plunger with it, it wouldn’t fit back in the injector.
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CobyRupert
Super Member
Ah, I see that the compression ratio is just a function of the volume ratio at bottom of stroke to top of stroke.
I guess I’ve always wrongly thought of it in terms of atmospheric pressure. That if you have a 10:1 compression, that cylinder would be at around 147 psi (absolute) at top stroke. (where atmosphere air pressure is 14.7 psi (absolute).
But that can’t be true with a throttle plate or a fast spinning motor that has restrictions in the air intake pathway and through valves. At bottom of stroke there must always be a bit of vacuum in the cylinder. This “baseline” vacuum increasing with rpm as you try to move air faster (or engine has a throttle plate.) This baseline vacuum pressure is then subjected to the compression ratio on the upstroke.
I guess I’ve always wrongly thought of it in terms of atmospheric pressure. That if you have a 10:1 compression, that cylinder would be at around 147 psi (absolute) at top stroke. (where atmosphere air pressure is 14.7 psi (absolute).
But that can’t be true with a throttle plate or a fast spinning motor that has restrictions in the air intake pathway and through valves. At bottom of stroke there must always be a bit of vacuum in the cylinder. This “baseline” vacuum increasing with rpm as you try to move air faster (or engine has a throttle plate.) This baseline vacuum pressure is then subjected to the compression ratio on the upstroke.
Bavarian
Platinum Member
You are quite right with the remaining vacuum at bottom dead center. And there comes the valve timing into play. The intake valve is left open for a few more degrees to take advantage of the flow of mass and the cylinder still keeps on filling even when the piston is already in a slight upward motion.
To your pressure calculation, theoretically you're right here as well. But if you measure the combustion pressure, you will see higher numbers than that. Every compression process creates heat. Heated air expands and creates more pressure. A process that is neccessary for the ignition of diesel fuel. Not so much of an advantage in gas engines. At some point, misfires are the result.
To your pressure calculation, theoretically you're right here as well. But if you measure the combustion pressure, you will see higher numbers than that. Every compression process creates heat. Heated air expands and creates more pressure. A process that is neccessary for the ignition of diesel fuel. Not so much of an advantage in gas engines. At some point, misfires are the result.
Raul-02
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Me too but they aren't
LouNY
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I don't know what you are referring to naturally aspiration just means it does not have a turbo or super charger to push AIR into the engine,Me too but they aren't
a NA diesel still has Injectors.
My two stroke diesel has injectors for fuel.
