Compression Ratio

[Jerry/MT]

I would be surprised if there were enough fuel pressure to overcome combustion chamber pressure during combustion, wouldn't that cause combustion in the injector lines and/ or injector pump... maybe because of the lack of air, but what about while purging the air if you lose the prime? ...again not a diesel mechanic. I will definitely be following this thread, there will undoubtedly be much to learn from here.

Compressing a gas causes its temperture to rise significantly. Not so wiith most liquids. The reason the diesel fuel ignites is because it's sprayed into a hot gas.

 

[MHarryE]

Think of how much the turbo boost pressure can be. That is an addition to compression. Our Allis-Chalmers engines in our largest combines would run about 24 psi boost at max power, on top of sea level atmospheric pressure of 14.7 psi pushing air into the combustion chamber. Actually direct injection is quieter than indirect injection. I had a BMW 328 TD indirect injection when I lived in Europe for a few years and the 320 TD direct injection that replaced it ad more power, better economy, and was quieter. Same with Kubota engines - my L5740 has an indirect injection engine but the direct injection version of the same engine has about 20% better economy, has more power, and is quieter than mine according to Kubota's information on their engine web site. According to my textbook the advantages of indirect injection are lower fuel injection pressure required(lower cost injection pump and injectors) and ability to use a wider range of fuels (run crappy fuel with fewer problems) at the expense of higher specific fuel consumption. My book gives a typical peak combustion pressure for a non turbo diesel of 830 psi for direct injection and 610 psi for indirect injection. Turbo engines are only limited by the amount of air you can push in until something gives, and over fueling only really hurts economy and emissions. Back in the old days at Allis-Chalmers we over fueled because everyone wanted more power and fuel was relatively cheap. Finding combines in our test fleet was often searching the horizon for the plume of black smoke. Looking at the chart in my book it looks like diesel will ignite at about 500 psi at normal temps but a lot depends on the combustion chamber design. When Deutz bought us and we had to switch to Deutz engines, we had to make ether starting aid standard and had it thermostatically controlled to come on at about 45 degrees F. The Deutz engine ran a very high boost pressure so it's compression ratio may have been low resulting in hard starting - been many years and I can't remember for sure.

 

[94BULLITT]

Anymore it seems like the compression ratio is dropping on diesel engine and going up on gas motors, or at least in automobiles. Mazda has gas motor with 13.0:1 and diesel with 14.0:1.
MAZDA: SKYACTIV-D | ENGINE | SKYACTIV TECHNOLOGY

 

[Mace Canute]

............Pre-ignition is the very method that a diesel uses to ignite the fuel..........

A diesel ignites the fuel by heat of compression. and the ignition point is dictated by when the fuel is injected into the cylinder. You cannot have PRE-ignition in a diesel engine. This should be obvious since there is nothing to ignite before the fuel is injected.

 

[Soundguy]

A diesel ignites the fuel by heat of compression. and the ignition point is dictated by when the fuel is injected into the cylinder. You cannot have PRE-ignition in a diesel engine. This should be obvious since there is nothing to ignite before the fuel is injected.

except on maybee a 353 whith a choke plate on, gaskets sucked in and drinking her own juices but that's a weird one.. ( weird scarry one! )

 

[Ken]

except on maybee a 353 whith a choke plate on, gaskets sucked in and drinking her own juices but that's a weird one.. ( weird scarry one! )

The old 310 or 610 gm engines with blower seal leaking didn't need diesel the engine oil was good enough until oil was gone.
ken

 

[Soundguy]

deffinately some scarry runaways. that and natural gas leaks!

 

[SPYDERLK]

A diesel ignites the fuel by heat of compression. and the ignition point is dictated by when the fuel is injected into the cylinder. You cannot have PRE-ignition in a diesel engine. This should be obvious since there is nothing to ignite before the fuel is injected.

:thumbsup: Ya-ay! ... Also, as comment to some previous posts: Beyond minimal compression for ignition the factor of the speed of that compression is important. Heat is being conducted away. A slow cranking low compression diesel may need some help getting going.
larry

 

[Jerry/MT]

A diesel ignites the fuel by heat of compression. and the ignition point is dictated by when the fuel is injected into the cylinder. You cannot have PRE-ignition in a diesel engine. This should be obvious since there is nothing to ignite before the fuel is injected. .


Mace,
You're right, detonation is would probably be a better term. Since there is no ignition system on a diesel there is no such thing as "pre-ignition". However my point was that the same phenomena causes both physical situations. It's just not desirable in a SI engine and it's necessary in a CI engine. Actually there is a time delay between injection of the fuel and the ignition point since some reactions have to take place before oxidation occurs.

 

[Mark Page]

Compared to a gasser how hot does the combustion chamber get on a diesel. I believe the flash point for diesel in atomized form is around 480 degrees.