According to this Bosch page, their glow plug was developed in the early 1920's.

Yet the diesel engine was around for at least 20 years prior to that; the French used it in a submarine in 1904, for instance.

Given that diesel technology relies on auto-ignition, how did they get the engine up to temperature without a glow plug?


Glow plug is merely an aid for starting the engine. There are many designs still produced today without any glow plugs (eg military diesels run without any electricity), and even many modern diesel engines can be started if glow plugs fail (unless onboard computer prevents that).

Diesel's auto-ignition comes from heat generated by adiabatic compression of air just before injection. Glow plugs just add a little heat so you need less cranking before compression heats the engine up into operating temperature.

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    IIRC, the D9 Caterpillar dozer my father used to drive when I was a kid would use an axillary gasoline engine to first, circulate coolant through the diesel engine which it would warm as it would warm up, then when the much larger diesel engine was warmed, would provide the starting power to turn the diesel engine over and get it started. This was very efficient for cold weather starting. Point is, no glow plugs involved. – Pᴀᴜʟsᴛᴇʀ2 Apr 1 '16 at 10:29
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    @Pᴀᴜʟsᴛᴇʀ2 Yeah, I think it was quite common for large diesels to be equipped with small gasoline engine as a starter motor. – Agent_L Apr 1 '16 at 14:54
  • The air is compressed, then fuel injected - there is no fuel-air mix... – Solar Mike Apr 11 '20 at 7:52
  • @SolarMike Well, there is when the fuel is injected, but you're right - I've fixed the description. – Agent_L Apr 12 '20 at 12:50

When I was little, my dad had a yacht which had been a northern European coastguard patrol vessel. He bought it as surplus military and converted it into a yacht. The engine, a Stork Werkspoor diesel, well that was quite a story. It was massive, almost 6' tall, with a gigantic flywheel. Each cylinder had a shelf in the casting, on which you placed a blow torch. They had to be lit about half an hour before you wanted to start the engine. When the individual cylinder heads were red hot my dad stood on a handle and pushed himself down from the roof, taking real good care the flywheel was positioned in just the right place, or it would kick back (I tried, it almost launched me through the roof). That engine drove a massive 5 blade prop with variable pitch, so no gearbox, you adjusted speed just like an aircraft, by changing the pitch of the prop.


I've not seen it done personally but I've been told that some old (WWII) military engines would be warmed up prior to starting in any number of ways which anecdotal included the use of lit marine flares.

Apparently some Russian tanks with Diesel engines would be started by cranking them with Petrol engines or even warming them over a fire.


To expand on the answer above, many old vintage diesels ran a 'hot bulb' design. There would be a section of the head where an open flame would be used to heat it, so the engine could ignite the lower grade fuel oils/diesels without the need for extremely high compression.

  • Hot bulb is not a diesel. For hot bulb, it's essential to reuse heat from previous stroke. True diesel ignites from adiabatic compression alone. – Agent_L Apr 1 '16 at 9:00
  • @Agent_L - I would disagree with you that a hot bulb is not a diesel engine. You could make the same argument against an indirect injected diesel engine. I think I get your meaning it is not what Rudolf Diesel invented, but it's main ignition source still occurs from the compression, then the injection of air into the air/fuel mixture to further the combustion process. Maybe it is you are saying just because an engine uses diesel as it's main source of fuel doesn't make it a diesel engine ... I get that as well. I would consider the "hot bulb" design as the for runner to the modern diesel. – Pᴀᴜʟsᴛᴇʀ2 Apr 1 '16 at 10:20
  • @Pᴀᴜʟsᴛᴇʀ2 Runner or ancestor - yet. But on Wikipedia it's explicitly mentioned "The hot bulb engine is often confused with the diesel engine, and it is true that the two engines are very similar". I believe that the source of ignition is the determining quality of a "diesel". – Agent_L Apr 1 '16 at 15:01
  • @Pᴀᴜʟsᴛᴇʀ2 Indirect-injection is also unclear distinction, as it encompasses multitude of superficially similar designs. Prechamber can be merely used as a mixer, a precombusting mixer, or it can be a source of ignition, as in hot bulb engine. Hot bulb undeniably is a prechamber design, but I stand in the opinion is that it's not a diesel, just because it can't ignite on the compression alone. – Agent_L Apr 1 '16 at 15:07
  • @Pᴀᴜʟsᴛᴇʀ2 here, from Mr Diesel's patent google.com/patents/US542846 : "At point 2 the mixture is ignited and by the now encrease of pressure is produced, accompanied by a very considerable increase in temperature". His patent was granted even though the hot bulb engine was patented in a previous year. The timeline is that if one is subtype of the other, it's the newer diesel that's a subtype of the older hot bulb - not other way around : ). – Agent_L Apr 1 '16 at 15:23

One method that is still used - but only on vehicles with metal air filter canisters is to remove the filter and put burning paper there - the hot air makes all the difference. Once running put the filter back in...

Another method is to heat the air inlet pipe with a blowtorch again : metal not plastic...

And yes I have seen both : heavy goods, agricultural and forestry are likely candidates...


I don't know what was used before glow plugs. But you can use a hair dryer to blow warm air into the air inlet of a diesel motor and it will make it start easily in cold weather. I've done this many times and it works like a charm.


If the compression ratio is high enough (around 22-1), there will be enough heat from the compression. Most diesel engines are turbo charged to increase power. The compression ratio is lower because the air is partly compressed by the turbine, but not while starting, so the glow plugs provide the extra heat needed.

  • Not always, trying to start cold engines in the middle of winter means the temperature rise due to compression is just not enough... see my answer above. – Solar Mike Apr 24 '19 at 8:18

A long, long time ago. I heard that a wick was lit and inserted into the combustion chamber. There must have been an access plug. The glowing wick provided a hot ember to ignite the air/fuel mixture for so many cycles before it was consumed and blown out the exhaust. An old Greek named Donias whose grandfather was fisherman told me this story as a memory from his youth.


I have a Volvo Penta md7a in my sailboat that starts right up with no need for glow plugs or preheating. I can even hand crank it should the batteries die by using a decompression lever that hold the valves open so you can get the substantial flywheel up to starting speed. Air compression and a squirt of fuel is all that’s needed.

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