Compression Ratio

[Mark Page]

I've noticed recently that not all diesel engines have the same compression ratio. I find this somewhat confusing. If all diesel fuel has a cetane rating of 45 wouldn't all engines need the same amount of heat to ignite the fuel air mixture. Higher compression ratios would seem to cause pre-ignition and lower ratios imcomplete ignition, especially in the winter. I've never really paid attention to this as the 3 diesels I've owned ran just fine at 18/1. I've seen 16 to one but never owned one.
What am I missing?

 

[mikehaugen]

That is an interesting question, I have never thought about it that way. I am no diesel mechanic but I would assume (to answer one of your questions) that you wouldn't get incomplete combustion from a lower compression because if you could even get some combustion that would increase the chamber pressure enough to ignite the rest.

the compression ratio isn't the only thing that will dictate chamber pressure, as valve timing and condition of rings will also. Air/ fuel ration may even play some part in how much pressure it takes to ignite. Again, this is all assumption- hopefully someone smarter will chime in.

 

[crusty369]

Compression on my new Mahindra 3616 Mits motor is 22.5/1. Think u are referring to air/fuel ratio. Diesel has a much higher ignition point than gas, and burns much hotter when it gets going. Also, temp remains consistent, as opposed to gas, hence the lack of spark plugs.
That's what always axed me on these tractors: he spec always shows hp, not torque. Torque numbers are what is important. That's what moves the dirt.

 

[greasemonkeyok]

I believe there is a difference based on the timing of the injection of fuel into the cylinder. MB & VW inject fuel directly into compressed air (piston is near the top of the stroke) and continue to inject as the piston moves back down during the burn. This creates more of a sustained "push" all the way down rather than one "bang" at the top of the stroke.

 

[mikehaugen]

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.

 

[stevem5215]

The combustion chamber shaping is the difference. Remember back to when the first CVCC Honda engine was released ? They started this all.

Steve

 

[greasemonkeyok]

I would be surprised if there were enough fuel pressure to overcome combustion chamber pressure during combustion.

Be surprised. The common rail fuel system in the 2009 / 2010 VW TDI vehicles uses a HPFP that can deliver fuel at over 25,000 PSI. It ain't your grandfather's diesel injection system.

This is from VW literature relative to the PD turbo diesel, so believe it or not:
"To accomplish the task of producing sufficient power while meeting emissions standards, the "PD" technology injects fuel directly into the combustion chamber at pressures up to an incredibly high 2,050 bar (30,000 psi). The purpose of the high pressure is to promote fine atomisation of the fuel which supports more complete combustion. To reduce noise, the engine employs a "pilot injection" system which injects a small amount of fuel prior to the main injection."

Compression ratio: 18.5:1.

 

[Soundguy]

16.8:1 on old ford red tiger diesels very late 50's era.. etc.


soundguy

 

[bigdeano]

Indirect injection engines have a higher compression ratio, like 22:1. They have a small combustion chamber where the fuel is injected, then the burning fuel shoots out into the main chamber. I think it's so they run quieter without the hammering noise. A little googling will tell you.

 

[Jerry/MT]

Mark,
As you probably know, compression ratio is a volume ratio and it's an important cycle parameter for any IC engine. It has a strong influence on the ammount of work done per unit air flow. In general, the higher the compression ratio, the higher the peak gas tempertaures and the higher the thermal efficiency. The min threshhold of diesel compression ratio is probably around 13-14 and the max is somewhere like 24-25. The lower end is dictated by the ability of the compression process to ignite the fuel and upper end by the material limits of the piston crown and the crown cooling. For tractors, weight is not a draw back in an engine but it is for over the road vehicles so engine weight is also a factor that influences the compression ratio. With increasingcompression ratios the loads on the pistons and sleeves, rods, crankshaft, and bearing increase with compression ratio also and that drives more weight into the engine. Thanks to the lastest EPA rules, emissions will also play a factor in compression ratios. the higher gas temperatures mean the more NOx is produce and the EPA doesn't like that.
Pre-ignition is the very method that a diesel uses to ignite the fuel. It's a good thing in diesel. It's just the opposite of a spark ignition engine. That's what the cetane number is all about. The higher the cetane number the easier it is to initiate combustion of the fuel.

Throw it all together and the 17.5+/- compression ratio is what you normally see in larger tractors(>~50 hp). In some of the smaller tractors (<~50 hp) you'll see higher compression ratios most likely because they have to get more work out of a smaller displacement and it's inherently harder to do on a smaller scale efficiently so the compression ratio needs to be higher.

Does this answer your question?

 

[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.