Minimum amount of compression to fire a diesel - SailNet Community
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Old 02-24-2010 Thread Starter
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Minimum amount of compression to fire a diesel

Anyone have a formula for figuring the minimum amount of compression to make a diesel fire? My volvo md7b won't fire up and I have had the injectors tested and they came out ok. After trying for the last few weeks to get it compression tested I have given up and ordered a tester so that should be here next week. I have gotten some vague numbers from other people saying that it should fire with 250psi. The manual does not give a minimum but it does say the starter speed compression should be 284 to 355psi. I am not real optimistic that I will get that given the engines age.

Alternately if someone had a formula for the temperature gained by compressing a given volume of air to a certain psi I could get a semi decent number based on the autoignition temp of diesel of 410 F.

Really all I want is a number so that when I get a reading I know if its reasonable. If its a reasonable number I will look at the fuel pump. If its not reasonable then I will have to pull the head off and look further.

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Old 02-24-2010
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If the manual says that comp should be between 284 and 355, then that is the range to aim for. Remember that fuel and temp will also play a role in whether an engine will kick. A tired engine with old fuel on a cold day will be crankier than the same engine with fresh fuel in the middle of august.
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Old 02-24-2010
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There is no set number at which it will fire. You are loosing a lot of heat to the block and to the exhaust as you try to start the engine which are two variables that can't really be accounted for. Use the number given in the manual as a guideline.
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Old 02-24-2010
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for every one psi of air compressed the temp rises 2 degrees Fahrenheit. So if you had 250 psi of compression the temp of the compressed air in the combustion chamber would be 500 degrees.
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Old 02-24-2010
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Quote:
Depending on the fuel grade, the auto ignition temperature of diesel fuel is 500-700 degrees Fahrenheit. A compression pressure of 300 psi should be considered minimum, especially in lower ambient temperature conditions.
I had to go look but, that is right from the textbook.
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Old 02-24-2010
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If temp is a direct ratio from the change in volume 22 to 1 then the instantaneous temps would be over 1,000f, enough for flash point of diesel....

Eric Roline
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Old 02-25-2010
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Quote:
Originally Posted by ericroline View Post
If temp is a direct ratio from the change in volume 22 to 1 then the instantaneous temps would be over 1,000f, enough for flash point of diesel....
The temps can reach over 3000 deg F during ignition then get much cooler during the scavenging period and intake stroke. The injectors meter the fuel into the cylinder usually several injections per cylinder firing. Making the engine run smoother and lowering emissions.

When the engine is cold things are much different, most of the heat generated by compression and friction in the cylinder gets absorbed by the cold metal of the cylinder. That's why diesels are a little fussier when cold. You have to crank them for a bit to warm the cylinder for the fuel to ignite. A lot of manufacturers overcome that problem with glow plugs or intake heaters to warm the in coming air so the engine will start easier/quicker.
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Old 02-25-2010
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Do you have white smoke coming out your exhaust when your cranking now? if so...

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Before ya go pulling the head off....just dont use your preheaters with it...and a very tiny bit....she will start.

IF no smoke you have a fuel issue....as in not getting any...and you need to figure out why then bleed your fuel system of air.

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The purchase price of a boat is just the admittance fee to the dance...you still have to spend money on the girl...so court one with something going for her with pleasing and desirable character traits others desire as well... or you could find yourself in a disillusioned relationship contemplating an expensive divorce.

Last edited by Stillraining; 02-25-2010 at 11:03 PM.
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Old 02-26-2010
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You beat me to it Still. When you pulled the injectors, were they wet or dry? Have you bled the fuel system and are you using fresh fuel? If you are getting fuel to the injectors and you a get white smoke while cranking try using a heat gun directed at the air intake so the combustion air is warm, should start.

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Old 02-26-2010 Thread Starter
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Everything was pretty wet when I pulled the injectors out. I have replaced the filters and bled the fuel lines. I may pull the fuel pump off and have that checked for pressure. It was putting fuel out.

I will have to rebleed everything when the injectors go back in assuming I have some compression. I will pull of the valve cover and gap those. I have been trying to avoid things that may take new gaskets until the very end.

I have been considering pointing a space heater at it for a few hours and getting the whole block warmed up a bit. Its been oddly cold here in florida for the last couple of months.

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