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?