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Old 22-06-2015, 16:06   #1
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What Is The Minimum Compression Ratio For A Diesel Engine To Operate

ie; What is the lowest possible compression ratio to actually achieve compression ignition. Difference between a cold or warm engine etc.
its something I've never even thought about before but I was reading one of those endless threads where someone is going on and on about whether they have to get a new engine or not because the compression has dropped but they never actually seem to have any figures or in some cases it turns out they havent done an actual test, they just think that the comp. has dropped out. Oops, turning into a rant...
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Old 22-06-2015, 16:57   #2
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

I dunno but I also would like to know. As well as enviromental conditions you mentioned, it depends somewhat on the speed of cranking, at least in a worn engine.

Of course most compression tests only give the pressure obtained during cranking rather than the actual ratio but it is the pressure that gives the necessary heat to initiate combustion. A leak down test is even more imformative as it helps determine where the problems lie.

But back to your question, what pressure is necessary for a diesel to fire up? Maybe google knows but I enjoy the answers from CF more
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Old 22-06-2015, 17:41   #3
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

The Perkin's 4.236 has a compression of 16:1.
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Old 22-06-2015, 18:00   #4
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

Typically a new engine should be around 450 lbs. minimum although many are over 500 lbs. The figure often used for the minimum compression on an older engine is 270 lbs but I'm sure engines would run below this because mine did and it was around 200 lbs.
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Old 22-06-2015, 18:39   #5
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

It depends on air temperature, engine temperature, cranking speed, whether or nor direct or indirect injection and whether turbocharged or naturally aspirated so sorry, no difinitive answer. Most engine overhauls I have performed due to not starting from low compression occurred in the winter months in an area where the seasonbla variation was only around 15 degrees.
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Old 23-06-2015, 01:06   #6
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

One of the reasons the modern diesel runs so smoothly is the low comp/ratios about 14.1 now used in conjunction with other trick devices such as multi phase & high pressure injection , variable exh valve timing all controlled by a little black box. Back in the 1920s thru to the 60s some Ag & marine engines had manually operated hi/low compression change over valves fitted to the Head, start in Hi position then change to low once running smoothly.
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Old 23-06-2015, 01:21   #7
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

Quote:
Originally Posted by shakey doug View Post
One of the reasons the modern diesel runs so smoothly is the low comp/ratios about 14.1 now used in conjunction with other trick devices such as multi phase & high pressure injection , variable exh valve timing all controlled by a little black box. Back in the 1920s thru to the 60s some Ag & marine engines had manually operated hi/low compression change over valves fitted to the Head, start in Hi position then change to low once running smoothly.
Doug, that's very interesting! But, aren't those very low CR engines turbos? That ws the impression that I had, but then you are the diesel guru. And I thought that in naturally aspirated engines the very high volumetric efficiency generated by the high CR was one reason for the excellent efficiiency of diesels compared to petrol engines. Is that correct?

And for the OP, you need to distinguish between compression ratio and compression pressures. The ratio is simply the volume of combustion chamber plus the cylinder at BDC to the volume of the combustion chamber alone. This ratio does not change as the engine ages. Compression pressures are actual measured pressures in the combustion chamber generated by the decrease in total volume as the piston rises up the bore. That pressure does decrease as the engine ages due to leakage past rings or valves or gaskets. As others have said, this typically is on the order of 500 psi when new decreasing to sub 200 when it will be hard to start and loosing some power once running.

Cheers,

Jim
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Old 23-06-2015, 04:19   #8
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

Jim.Yes you are right re the forced induction, I elected to only mention a couple of reasons due to the lack of time available, still trying to gather a quid.
A combination of the Otto & Atkinson Principles.
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Old 23-06-2015, 09:08   #9
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

I would also recommend having your own tester and check after each oil change or two, so you have a control pressure reading to compare to. Log your findings.
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Old 23-06-2015, 09:51   #10
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

Quote:
Originally Posted by Jim Cate View Post
Doug, that's very interesting! But, aren't those very low CR engines turbos? That ws the impression that I had, but then you are the diesel guru. And I thought that in naturally aspirated engines the very high volumetric efficiency generated by the high CR was one reason for the excellent efficiiency of diesels compared to petrol engines. Is that correct?
You are correct. The fundamental physics dictate that the compression ratio is linearly related to its fuel efficiency. That is to say, the higher the CR, the greater the fuel efficiency. This applies to all engines, not just diesel. Since gas engines have a CR around 8-10, you can see how diesels are fundamentally more efficient.
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Old 23-06-2015, 11:20   #11
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

This is an interestingly thread and I'm encouraged about the possibility of a diesel firing with a cylinder at 200 or so psi as I have atm.
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Old 23-06-2015, 11:25   #12
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

Just a little side thing, expansion ratio is the real thing that is important in efficiency, it determines how much energy is extracted from the combustion products, I'm always surprised we haven't seen more engines with limited compression by late valve timing or such to reduce compression but allow higher expansion ratios. I've got ideas for building a tiny diesel in the 25 cc range. The controlled injection of a partial drop of fuel is the problem as I see it, I think it could be solved with a pressurized fuel tank and electronic injection.
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Old 23-06-2015, 11:51   #13
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

My Perkins 4-236 with less than 500 hours on it was about 340-345 PSI.... all cylinders.
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Old 23-06-2015, 12:50   #14
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

so - somewhere between 200 - 500 psi and the higher the figure sort-of the healthier the engine ( bearing in mind all the caveats - cranking speed, temp etc). Shame its such a PITA to pull the bloody injectors and do a test, be good way to keep an eye on internal engine health
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Old 23-06-2015, 12:52   #15
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

Yeah, not only pull the injectors, but have an adaptor made, cant get a good reading on diesel compression just hand holding the gauge in the hole.
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