All Internal Combustion engines use a flammable fuel (liquid or gas) source. The primary difference is in how they use the fuel.

In the realm of stationary engines, there are basically two kinds of engine, those which compress and ignite the fuel with a spark or heat source and those which rely solely on the heat generated from highly compressing the combustion mixture for ignition. The later kind are a class of Internal Combustion called Diesel Engines.

The "combustion mixture" is the mixture of fuel and air within the combustion chamber which ignites and explodes to generate the power to run the engine. With a liquid fuel engine the fuel is injected into the chamber along with air, or is pre-atomised in a carburetor then sucked into the combustion chamber. Liquid fuels have to be atomised as they will not burn efficiently (or in some cases at all) without mixing them with a large quantity of air rich in oxygen. Gaseous fuels are usually piped directly into the air intake as gas naturally mixes to fill all available volume.

There are four primary fuel types, and all fuels fall into these types:

Fuel Category Common Fuels Suitable Ignition Source
Unrefined or Heavy Oil Crude Oil Hot Bulb
Hot Tube
Refined Oil Diesel
Vegetable Oil
Hot Bulb
Hot Tube Igniter Make and Break / Wipe Spark
Compression Only
Compression with Glow Plug (essentially a more recent Hot Bulb)
Highly Refined Oil Power Kerosene
Petrol / Gasoline
Vegetable Alcohol
Aviation Kerosene (AVTUR)
Aviation Gasoline (AVGAS)
Spark (Coil or Magneto)
Gas Liquid Petroleum Gas (LPG)
Pilot Flame
Spark (Coil or Magneto)



Ignition Source Ignition Type Description                                                Comment
Hot Bulb Heat A blow torch is set so that its flame plays over a protrudence on the head of the engine known as a bulb. Once the bulb was hot enough (cherry red) the engine could be started by turning its flywheels. The compression of the combustion mixture combined with the heat stored in the head of the engine caused the fuel to ignite/explode commencing the working cycle. The blow torch was only required on start up or during very light load, once working hard, the engine would stay hot enough to keep running without the torch. Long start up time.
Flame Heat / Flame This was a mechanically complex form of ignition. A pre-combustion space would be charged with combustion mixture at the same time as the main combustion chamber within the cylinder. An open flame from a torch or jet was directed into a portal into the pre combustion chamber (which is closed to the main cylinder) igniting the small amount of mixture there. This portal would close just before TDC and another portal between the pre combustion chamber and the combustion chamber would open igniting the main charge. Imagine timing an engine like this! Hard to time. Hard to keep running
Hot Tube Heat / Flame The Hot Tube is really a progression from the Hot Bulb concept, although it did allow some adaption of the ignition timing which was impossible with hot bulb. A flame from a jet is directed into an insulated chamber or "chimney". Within this chamber is the hot tube itself. The Hot tube is open to the combustion chamber but closed to the chimney. The sole purpose of the chimney is to retain the heat of the tube. As the combustion mixture is compressed some of it is forced into the hot tube where it ignites, thus igniting the main charge.

Hot Bulb engines can only run well at one speed, Hot Tube engines can have their speed (and necessarily timing) changed by changing the length of the tube.

Hard to time. Hot Tubes are easy to destroy due to the balance of the requirement of keeping the tube hot combined with the forces it is exposed to. Too hot and it breaks during firing.
Compression (Diesel) Heat of Compression The combustion mixture is compressed within the combustion chamber until it ignites from the heat generated as the mixture is compressed. Compression engines (diesels) are required to compress the combustion mixture much more than other ignition style engines. The fuel is nearly always injected directly into a compression engine.

To make these engines easier to start from cold, most compression engines have some way of releasing the pressure built up in the cylinder(s) to allow easier cranking of the engine until it reaches a speed where the compression release can be closed and the engine start. The pressure relief is usually a small tap or opening, or in more modern examples the exhaust valve is held slightly open so no pressure is built up.

Can be hard to start.
Compression with Glow Plug Radiated Heat The glow plug is a more modern version of the Hot Bulb engines, and is very similar to either hot tube in principal. The difference is that the combustion mixture does not enter the glow plug. The glow plug provides a hot are within the combustion chamber to aid ignition. Originally glow plugs were heated with a torch, but in the more modern compression engine (such as those found in cars and trucks today the glow plug is electric). Does not have the disadvantages of the Hot Tube.
Igniter - Wipe Spark Spark The forerunner to the Spark Plug the Igniter is a grounded tube with a set of points much like those found in a car (mechanical ignition) except that those points are within the combustion chamber.

The Wipe Spark Igniter uses a low tension electrical source such as a battery. To generate the spark, a contact is quickly wiped over another one effectively shorting the battery, at the completion of the wipe when the contacts separate a spark leaps from one to the other causing another spark to arc across the igniter points within the engine.

The platinum points within the combustion chamber quickly wear and pit requiring replacement.
Igniter - Make and Break Spark The Make and Break Igniter is almost identical to the Wipe Spark Igniter except that it uses a higher voltage generated by a low tension magneto or battery and and buzz coil. Ditto
Spark Plug Spark The Spark plug is similar to the Igniter except for one major difference. The method of generating the spark is outside the combustion chamber. A high tension magneto or buzz coil and battery) are used. Spark plugs operate with a much higher voltage than the Igniter which allows steel to be used for the spark terminals within the combustion chamber which does not deteriorate like the platinum of the igniter points. Requires more complex ignition system than any other form of ignition, but since it offers both longevity and control of ignition which the alternatives do not, it is the ignition method prevailing today.


If you are looking for more detailed information on ignition, and how it all works, I strongly recommend Harry Matthews Old Engine Magneto pages.

This email address is being protected from spambots. You need JavaScript enabled to view it. recently wrote to me about this article, and commented that...

Hi Paul,
            I have just been perusing your page and I question your article, describing Aviation Kerosene as being AVGAS.

, is Aviation Gasoline (petrol), for aviation piston engines.
AVTUR, is Aviation Turbine Fuel (power kerosene) for aviation turbine/jet engines.
I like your page for its nostalgic value (I was a school kid on a farm (Wagga Wagga, NSW)), before joining the armed forces (21 years' service).

In response Mick is both write and wrong - like everything in life there are differences... I thought I'd post this to clear it up (I hope!)...

Hi Mick,
Actually you are both right and wrong. They are both petroleum distilates and very similar to each other - just slightly different process in the cracking and the additives put in post refining. Aviation Kerosene (actually Kerosine to be technical :) is based on lighting kerosene not power kerosene. "Power kerosene" of yesteryear really is much closer to what we call petrol today than what we call kerosene today. What is sold generally in shops as kerosene today is for lighting and in some cases heating.

To reduce confusion in the future, I've updated the table to include both AVTUR and AVGAS as highly refined oils.

There is a good article from BP which covers many of the differences, including how different they are around the world - interesting reading if you are a chemist :)

BP Air Fuel Handbook

Thanks to Mick for pointing out the inconsistancy!