Basic (stupid) questions about gas and diesel engines.

[ericm979]

The compression ratio spec is usually the mechanical compression ratio of volume at BDC divided by volume at TDC. There are a number of effects that can end up cramming more fuel/air into the cylinder than the mechanical ratio would indicate. There is a ram air effect in the intake where the mass of the air flowing in compresses the air in the cylinder slightly. In addition there is a resonant effect that is determined by the size of the intake tract. The opening of the intake valve causes a negative pressure wave to travel up the intake port and into the manifold. When it reaches a place where the cross section area increases (i.e. a plenum that feeds multiple cylinders) there is a positive wave reflected back down the intake tract. If it arrives as the valve is closing it stuffs more air/fuel mix into the cylinder. This effect is quite strong if the engine is designed for it. It's why two strokes with expansion chambers make so much power (it works on the exhaust too and the forces are stronger there). Modern LS engines have quite long intake tracts curled up into a log in the valley between the cylinders. Some '50s/60s Dodge performance v8s had two carbs one on each fender with long intake tracts crossing over into the opposite cylinders.

Last in a performance engine with overlap between exhaust and intake open periods there is a flow through effect where negative pressure on the exhaust side due to resonance pulls mix through the intake port. Taken to extreme this can make the engines run poorly at lower rpms where the waves are not in sync.

 

[MossRoad]

Me too but they aren't

naturally aspirated diesel at DuckDuckGo

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Natural aspiration refers to no turbo or super charger, not fuel injection.

 

[finn1]

Me too but they aren't

naturally aspirated diesel at DuckDuckGo

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Yes they are.

 

[i7win7]

I though all diesels were fuel injected. And all engines, both gas and diesel, that have fuel injection have to have a return line to the fuel tank.

Here's a simple 1 cylinder diesel with 4 moving parts piston, rod crankshaft and throttle

rc diesel engine conversion - Search Videos

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[Egon]

On some of those rc engines the downward stroke Of the piston will compress the fuel/air mixture in the crankcase. The compressed mixture then gets injected into the cylinder through fixed ports on the cylinder Walls. The mixture helps clear the exhaust gasses on the upward stroke.

 

[MossRoad]

On some of those rc engines the downward stroke Of the piston will compress the fuel/air mixture in the crankcase. The compressed mixture then gets injected into the cylinder through fixed ports on the cylinder Walls. The mixture helps clear the exhaust gasses on the upward stroke.

Isn't that the way most 2-strokes work? My motorcycle did.

 

[Egon]

Isn't that the way most 2-strokes work? My motorcycle did.

Yes, for sure.

 

[MossRoad]

Yes, for sure.

It's been a long time since I got into the bottom end of a 2-stroke. I used to work on my RD400 quite a bit, however, I stopped drag racing when we had kids. My chainsaws, weed eater, snow blower, leaf blower have never needed to be opened up in 35 years.

 

[LouNY]

My 2 stroke has injectors and a supercharger it's a little 4-43 screaming jimmy Detroit Diesel

 

[RoostersGarage]

To my knowledge there cannot be a carbureted diesel engine. As it's a compression-fired engine, having the air fuel mix in the cylinder as the piston is on the compression cycle, the fuel would ignite early and destroy the engine.

On a gas engine, the vacuum is the highest at idle and drops to nearly 0 at wide open throttle. On a carbureted engine the vacuum can pull the fuel and mix pretty well but once it's WOT it has to pump more fuel in because of the drop in vacuum. Most carburetors have some mechanism to dump fuel into the air stream when the throttle opens rapidly so it won't lean out and melt a hole in the piston. Some carburetors even have a mechanism on the throttle plate to take care of the rapid closing of the throttle at high RPM to allow the unusually high vacuum to open a valve in the throttle plate and allow a bit of extra air in. Apparently there was an issue with Mustangs on the Autobahn that brought this addition about.

A diesel can safely run a lean mixture but running too rich can actually melt the pistons. Some of these diesel "tuners" that think fuel adds power and they overfuel will actually destroy pistons and sometimes even the block with molten metal. This is why a leaking injector can totally melt down an engine.

On a gas engine at idle, there is very little of the rated cubic displacement of air going into the engine. Think of breathing through a tiny coffee stirrer straw. Even though your lungs can take in a lot of air, only a small amount will pass through the tiny hole. Given enough time to "intake" you could eventually pull it in, but an engine at 800 rpm only has a fraction of a second to bring the air in.