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Old 23-06-2015, 13:44   #16
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

Interesting thread... since I know next to nothing about diesels and regard mine with considerable suspicion.
let's say you have one of these old worn out low compression motors. What techniques can you use to get it lit off? Besides starting fluid?
Can you warm the engine in some way to get enough heat into it to fire? Heating pad? Heat gun? Propane torch (yikes!)?
Will thicker oil in the crankcase seal the worn rings enough to get cylinder pressures to the point it will go boom?
Can you pull the injectors and squirt a bit of oil into the cylinder before cranking it?
If you have compression releases, can you use them in some way during the cranking process? Maybe to conserve battery juice to spin it long enough to get enough heat and oil into it to fire? Or will the fuel just wash the oil off the cylinder walls and lower compression?
Or just squirt some WD40 into the intake and keep cranking?
And what difference would the presence of glow plugs make in the process?
Obviously, I'm talking about an engine that needs overhaul or cylinder head work. But if that's not an immediately available option? And you really must get the thing running somehow...?
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Old 23-06-2015, 15:21   #17
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

The reason for a compression test (in any type of piston engine) is to detect a possible problem. It's not to see how low it can go and still run. The manufacturer has specs like as has been suggested 450 psi. Three years ago my 10 year old 2GM 20 was hard to start so I borrowed a diesel compression gauge which is much more substantial than for a petrol engine. A test indicated low compression on one cylinder. This gauge printed out pressure on a slip of paper. I pulled the head off and partly filled each cylinder in turn with WD40. On the cylinder with low compression the fluid drained past the cylinder quickly. On dismantling it was found that the rings on that cylinder were broken and the piston damaged. The cylinder head was slightly warped and presumably had allowed some cooling water into the cylinder. The engine was then completely overhauled. The cylinders were then bored to the maximum of +60 thou and the head planed -8 thou. This combination naturally raised the compression ratio from (as I recall) 22:1. An adverse effect of this was that on shutdown it stopped with a violent kickback. As suggested by a cylinder head expert I increased the valve clearance from recommended 8 thou to 9.5 thou. The engine now runs beautifully. I revisited the water lock and installed a siphon break. The new valve clearance allowed slightly less air into the cylinders and slightly decreased the EFFECTIVE compression ratio.

As has already been suggested turbo engines have a lower MECHANICAL compression ratio. That is going by the swept volume ratio to the head volume. But because of forced induction they have a higher EFFECTIVE compression ratio as there is more air induced to compress.

Old piston engined aircraft such as DC6 and Lockheed Super Constellation have mechanical superchargers and the Constellation mechanical plus turbo chargers to maintain power at high altitude where the air is thinner and the effective compression ratio would decrease without supercharging.

So a lower than recomended compression reading indicates trouble with a capital T.
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Old 23-06-2015, 15:41   #18
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

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Originally Posted by GrahamHO View Post
The reason for a compression test (in any type of piston engine) is to detect a possible problem. It's not to see how low it can go and still run. The manufacturer has specs like as has been suggested 450 psi. Three years ago my 10 year old 2GM 20 was hard to start so I borrowed a diesel compression gauge which is much more substantial than for a petrol engine. A test indicated low compression on one cylinder. This gauge printed out pressure on a slip of paper. I pulled the head off and partly filled each cylinder in turn with WD40. On the cylinder with low compression the fluid drained past the cylinder quickly. On dismantling it was found that the rings on that cylinder were broken and the piston damaged. The cylinder head was slightly warped and presumably had allowed some cooling water into the cylinder. The engine was then completely overhauled. The cylinders were then bored to the maximum of +60 thou and the head planed -8 thou. This combination naturally raised the compression ratio from (as I recall) 22:1. An adverse effect of this was that on shutdown it stopped with a violent kickback. As suggested by a cylinder head expert I increased the valve clearance from recommended 8 thou to 9.5 thou. The engine now runs beautifully. I revisited the water lock and installed a siphon break. The new valve clearance allowed slightly less air into the cylinders and slightly decreased the EFFECTIVE compression ratio.

As has already been suggested turbo engines have a lower MECHANICAL compression ratio. That is going by the swept volume ratio to the head volume. But because of forced induction they have a higher EFFECTIVE compression ratio as there is more air induced to compress.

Old piston engined aircraft such as DC6 and Lockheed Super Constellation have mechanical superchargers and the Constellation mechanical plus turbo chargers to maintain power at high altitude where the air is thinner and the effective compression ratio would decrease without supercharging.

So a lower than recomended compression reading indicates trouble with a capital T.
Yeah, the Yanmar GM piston problems are well documented on CF. My mechanic didn't even need a compression test to know one piston was cracked on mine! The previous two kept wanting to do the valves.
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Old 23-06-2015, 16:35   #19
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

Cheechako; At age 76,(me that is) I'm hoping my reconditioned 2GM 20 will give me another 10 years service. Then I'll get a smaller yacht with an outboard. Off thread I'm sorry.
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Old 23-06-2015, 16:52   #20
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

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Cheechako; At age 76,(me that is) I'm hoping my reconditioned 2GM 20 will give me another 10 years service. Then I'll get a smaller yacht with an outboard. Off thread I'm sorry.
yeah... I hear you... it should! Hopefully they improved the pistons after the initial go around! A lot of people don't have issues....
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Old 23-06-2015, 17:43   #21
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

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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...
Depending on Cetane of fuel,it appears to be around 300-350 PSI to produce cylinder temp of approx. 500F to ignite diesel.
As usual-Depends....

My VP 2003 came with 16 or 16.5:1 CR. It doesn't take much valve degradation to make it very hard to start-especially when cold.

Here is a link to in depth study. Cheers/Len

http://transportation.centennialcoll...0fuels%204.pdf
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Old 23-06-2015, 18:04   #22
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

Charlie.


Slow speed & medium speed diesels have indicator cocks fitted to cyl heads allowing the fitting of a OKill gauge used to measure cylinder peak compression & firing pressures. When used inconjunction with a indicator card piston ring, valves and fuel pump wear etc can be detected. Recent built engines have transducers fitted making it far easier & quicker to collect data.
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Old 23-06-2015, 20:29   #23
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

Back in the day!when I worked on a lot of diesels and engines would come in as no starts and some techs would do a compression test right off and declare the compression to low and call for a rebuild I felt that this was not the 1st thing to do but the last ! the engine had most likely been cranked on a lot washing all the oil from the cylinder walls and a compression check is worthless even after putting oil in the cylinder which is an unrealistic situation although good for raising comp to help get it started there will never be that much oil in the cylinder under normal circumstances
my method was to test all the glow plugs,get the engine running no matter how, WD40, heat in the intake, etc, once running it should run on its own even if it is worn out, at that point you can access if you have a miss, injector problems, pumps, fuel related issues ,mechanical,etc
Once everything is correct fuel and mechanically,you can get many more hours beyond manufactures minimum comp ratio by advancing the timing ,cetane boosters,injector cleaners,40 wt oil, heat in cold weather, what ever it takes,ether! until a rebuild is a must
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Old 23-06-2015, 21:53   #24
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

the ratio for modern gas engines is between 1:10 and 1:15, for Diesel without turbo, compressor etc is between 19:1 and 23:1. These values are given by construction and you may find the info in technical manuals or so. There is nothing to measure. Compression pressure you can measure and would be 23 to 28 bar or less depending on construction of the Diesel.

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Old 25-06-2015, 07:08   #25
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

What no one mentioned yet is how dang important that diesel engines wear evenly. If you have a 4 cylinder that has 400# on 3, but 200# on one, it will have a far more drastic effect on the engine running. Having an even low compression worn engine, may run sweetly, but smoke and have a lot less hp than when it was new - but still run nice. Having a new engine with a single cylinder with low compression will affect the way it runs, especially at idle and throw off the harmonics of the engine.

So, when checking for compression, I am more interested in variance than looking for high compression.

To answer the OP, you can find what the manufacturer recommends for "in spec" compression numbers (high and low) in the tech manual for your engine. Check whether in spec with those numbers, but more importantly whether they are all within a small variance of each other.

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Old 25-06-2015, 14:28   #26
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Re: What Is The Minimum Compression Ratio For A Diesel Engine To Operate

[QUOTE=tdoster;1855338]What no one mentioned yet is how dang important that diesel engines wear evenly. If you have a 4 cylinder that has 400# on 3, but 200# on one, it will have a far more drastic effect on the engine running. Having an even low compression worn engine, may run sweetly, but smoke and have a lot less hp than when it was new - but still run nice. Having a new engine with a single cylinder with low compression will affect the way it runs, especially at idle and throw off the harmonics of the engine.

I absolutely agree. And if you find a compression variance it's easy enough to check the valve clearance and then pull the head off and check the valves. In my case I also poured a cup full of WD40 into each cylinder at the bottom of stroke and in one cylinder it ran quickly past the faulty piston. Another risk with hard to start motors is that over long cranking on the starter can fill the exhaust with water if you don't turn the water off until it fires. (stop it then start again so you don't lose an arm)
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