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Diesel engine starting

So explain to me just why a diesel is harder to start than a gasoline engine. Fuel gel? Isn’t there an additive for that? Oil gets too thick? Isn’t this the same for a gas engine? Compression not hot enough to “spark” the start? Glow plugs? I never quite understood the problem with this. Rocketman

Diesel is less refined than gasoline, and is heavier. It’s more prone to congealing when the temperature gets low. The glow plugs help with the initial combustion, but once a diesel does start it will keep running, Even if the alternator and battery fails.

If everything’s working right, they shouldn’t be hard to start. I think we get a little spoiled because gasoline engines will usually start with all sorts of problems.

Fundamentally, the issue is that diesel is a less explosive fuel and consequently there’s a narrower set of circumstances that has to happen to get it running. First off, they’re harder to turn over because of the higher compression involved so if the battery and starter aren’t in tip-top shape you get problems there. Secondly, because diesels rely on heat generated by compression alone, they are far more sensitive to lowered compression so things like worn rings and mis-adjusted valves will cause difficult starting. Cold temperatures also deter the heat generated by compression, causing hard-starting, although usually if everything else is working a diesel will eventually start without glow plugs, though the battery may not hold out.

In practice, though, when a diesel gets to the “start, ya bastard!” (an actual brand-name of a starting fluid in the UK and Australia) stage it’s usually a combination of all of the above, with maybe some fuel gelling or injector pump issues thrown in for good measure.

Are you talking about a problem or simply waiting for glow plug warm up and a slightly longer crank time?

There are no spark plugs since fuel is ignited by heat generated by compression (22.5 to 1 compression ratio) so the glow plugs warm the combustion chamber so the fuel can ignite. Thats it in a nutshell.

They really are not harder to start.  They are a little different however.

Yes fuel can gel, but only if you have fuel that has not been treated for the temperature.  This may happen if you fill up in Florida and drive to Ohio (yea, you can do that without stopping for fuel on the way, I have done it.  In Florida the fuel is treated but not likely to the same level as that sold up north. 

They have glow plugs and in order to start easily below +40?F they come on and depending on the temperature may stay on for a few seconds to almost a minute.  A light tells you when they are done. 

 At that point they should start right up.

 The coldest start I have had in a diesel was -31?F up in Windsor Canada in the late 1970s.  I was at a motel and I went out to leave and all I heard were gasoline cars trying to start, no one seemed to be going anywhere.   I got in my car, waited for the glowplug cycle and it started on the first try.  It did not run great.  For the first few blocks the best I could do was about 15 - 20 mph, but it was the only thing moving.  By the time I got to the bridge it was almost up to normal power.  

 My Rabbit diesel had a manual advance (injection timing), and if I had not used it, I doubt if it would have started. 

 So I guess I have quite a few miles in two different diesels of different ages and I don't understand the problem, I don't see that there is a problem with modern diesels.

meaneyedcatz is on to it. The fuel is completely different, and so is the means of combustion. Diesel fuel has a much higher flash point than gasoline, and also vaporizes at a much higher temperature. Plus, there is no spark plug to initiate the burn.

Diesel engines use compression only to make the burn possible. When you compress a gas (in this case air) in a sealed chamber, the temperature of that gas goes up. Once near TDC on the compression stroke, the air should be hot enough to burn the fuel. The fuel is forced in at high pressure (like a few hundred psi), which causes it to vaporize. The vaporized fuel in the hot compressed air burns readily. Gasoline by contrast will begin to vaporize at room temperature. Bump this to a couple degrees in a pressurized environment, and it will flash to vapor. Spark it and it will burn rapidly.

When the engine is cold, the block and heads try to absorb a lot of this heat, making it harder for the engine to start. Glow plugs help by pre-heating the combustion chamber, and preventing this from happening. But, they cannot get the combustion chamber up to ‘warmed-up’ temperatures. Also, the colder the ambient air, the harder it is to heat the air up to combustion temps.

When the engine is worn, the compression gets harder to build. Worn rings prevents the compression from building up to the levels the cycle needs to begin. Couple that with cold temperatures, and you may never get a worn out diesel to start. Once started, tho, the build up of temperature as it warms up can help compensate for the ring wear enough to keep it running.

Diesel fuel requires heat to ignite it rather than spark. Generating the heat from glow plugs requires a bit of battery power. Thats why most diesel engines have 2 batteries. Not all diesel engines have glow plugs though. Higher compressions also require heavier duty starters which also consume battery power.


Thanks folks! I never owned one . . . but see folks stuck once in awhile with diesel equipped engines . . . also see gasoline equipped engines, but I have a bit of experience with those engines. Very interesting . . . thanks for the responses. Rocketman

Very well spoken.