Why do some diesels require glow plugs and others don't?

[dcyrilc]

The title pretty much states my question.

What makes the difference on why a diesel engine requires glow plugs or not?

I have an old JD2240 which does not have glow plugs. I've been reading about start times on machines with glow plugs and some talk about taking 5- 10 seconds to start with the glow plugs. I have no heater and no glow plugs and even at 20 degrees last week it only spun 5-6 times before firing (about 3-4 seconds).

Just curious as to what makes mine start so easy without plugs when others don't.

Thanks

 

[Raspy]

Some diesel engines, and almost all earlier designs from the past, used a pre-combustion chamber design. This is a small chamber next to the main combustion chamber where the fuel is injected and the burn starts. Then a jet of hot burning gas passes into the main chamber and completes the burn.

This, I think came from the original "hot Bulb" design where there was a flame heated section of the head so the engine could ignite low grade oil with low compression.

Now days, the "open" chamber or direct injection design is the best and used by Cummins and others. The fuel is injected directly into the combustion chamber.

The smaller the chamber area the more surface area there is per volume of air. So the pre-combustion design has a lot of surface area per volume in the pre combustion area and, consequently the air cannot heat up as well. Or loses it's heat to the surrounding walls faster. The hot air has a shorter distance to the cold wall. This equals harder starting when the engine is cold, and thus the need for glow plugs. The glow plug is just a hot surface near the spray pattern that helps ignite the fuel when cold.

Some of these engines, like the old Mercedes 5 cylinder ones, have very high compression ratios of about 21 to 1. The Cummins is at about 17 to one. Even the lower compression ratio direct design will start better than the higher compression pre-combustion design. Yours is probably a direct injection system.

Cummins engines start very easily, but if cold enough, need help too. They have a grid heater that heats the incoming air in the intake tract, but no glow plugs.

 

[Hooked_on_HP]

I think a lot of it has to do with injection timing. My Ford 1900 has 21-1 compression ratio and injects directyly into the cyl. When the engine is cold you have to use the glowplugs for 15-30 seconds.When the engine is warm it fires right up. Every owner of the compact Fords I have talked to has to use the glowplugswhen the engine is cold.
Bill

 

[TGF]

I believe mine may have glow plugs, I have to turn the key to warm for 15 seconds before starting it in the winter time. Its a 2010 3038e.

edit --

Actually it may have an air warmer instead. I am looking through the shop manual.

 

[whistlepig]

Some diesel engines, and almost all earlier designs from the past, used a pre-combustion chamber design. This is a small chamber next to the main combustion chamber where the fuel is injected and the burn starts. Then a jet of hot burning gas passes into the main chamber and completes the burn.

This, I think came from the original "hot Bulb" design where there was a flame heated section of the head so the engine could ignite low grade oil with low compression.

Now days, the "open" chamber or direct injection design is the best and used by Cummins and others. The fuel is injected directly into the combustion chamber.

The smaller the chamber area the more surface area there is per volume of air. So the pre-combustion design has a lot of surface area per volume in the pre combustion area and, consequently the air cannot heat up as well. Or loses it's heat to the surrounding walls faster. The hot air has a shorter distance to the cold wall. This equals harder starting when the engine is cold, and thus the need for glow plugs. The glow plug is just a hot surface near the spray pattern that helps ignite the fuel when cold.

Some of these engines, like the old Mercedes 5 cylinder ones, have very high compression ratios of about 21 to 1. The Cummins is at about 17 to one. Even the lower compression ratio direct design will start better than the higher compression pre-combustion design. Yours is probably a direct injection system.

Cummins engines start very easily, but if cold enough, need help too. They have a grid heater that heats the incoming air in the intake tract, but no glow plugs.

I always wondered about that. My 1964 MF135 diesel would sit outside year round but would fire up in one spin of the motor in -10 degree weather. It didn't have glow plugs. My 2006 Kubota sits in a heated garage and won't start without warming the glow plugs in 90 degree weather for a few seconds. Still not sure if the Kubota way is better.

 

[Raspy]

Some Perkins engines have a fuel valve that, on the preheat setting, opens and leaks fuel into the intake manifold. A heated wire in the manifold lights the fuel on fire and then you start the engine which just sucks the fire right in. It takes about 30 seconds for the process.

Another way is to stick the flame from a burning propane torch into the intake and crank it. I've also used burning scraps of newspaper. :laughing:

Or just crank it faster by pushing the car and getting the cranking speed high enough that the cooling effect doesn't count so much.

 

[jenkinsph]

From what I can tell if it is direct injection it will start easily if it is indirect injection it may require glow plugs if cold.

 

[dcyrilc]

From what I can tell if it is direct injection it will start easily if it is indirect injection it may require glow plugs if cold.

I guess another way to ask this question might be to ask what are the advantages to designing an engine which needs glow plugs to start vs one like mine which doesn't need them. It seems like most of the engines I read about which have glow plugs are newer, smaller machines. Mine on the other hand is 30 years old and 50 HP PTO.

Could it have something to do with the size of the engine or perhaps an efficiency/emmissions thing???

 

[Willl]

Could it have something to do with the size of the engine

Couple of quotes from below link;

"the cold engine block acts as a heat sink"
"Their cylinders are large enough that the air in the middle of the cylinder is not in contact with the cold walls of the cylinder, and thus retains enough heat to allow ignition."

Glowplug - Wikipedia, the free encyclopedia

 

[Raspy]

I guess another way to ask this question might be to ask what are the advantages to designing an engine which needs glow plugs to start vs one like mine which doesn't need them. It seems like most of the engines I read about which have glow plugs are newer, smaller machines. Mine on the other hand is 30 years old and 50 HP PTO.

Could it have something to do with the size of the engine or perhaps an efficiency/emmissions thing???

Any diesel engine will, sooner or later, need a starting aid. Glow plugs are a common solution. Smaller displacement per cylinder means it's more likely to need help starting because of the short heat path to the cold cylinder walls. Pre-combustion engines are much more likely to need help.

I'm not a big fan of pre-combustion engines, but they might have the advantage of lower NOX and they have high combustion chamber turbulence. That doesn't mean they are necessarily cleaner burning though. My Mercedes diesels were not clean at all. The combination of harder starting, consistent modern fuel, higher compression ratios and less suitability for modern high performance makes them yesterdays technology as far as I'm concerned.