Model Engine - Diesel Engines

Diesel Engines

For more details on this topic, see Carbureted compression ignition model engine.

Diesel engines are an alternative to methanol glow plug engines. These "diesels" run on a mixture of kerosene, ether, castor oil or vegetable oil, and Amsoil cetane or amyl nitrate booster. Despite their name, their use of compression ignition, and the use of a kerosene fuel that is similar to diesel, model diesels share very little with full-size diesel engines.

Full-size diesel engines, such as those found in a truck, are fuel injected and either two-stroke or four-stroke. They use compression ignition to ignite the mixture: the compression within the cylinder heats the inlet charge sufficiently to cause ignition, without requiring an applied ignition source. A fundamental feature of such engines, unlike petrol (gasoline) engines, is that they draw in air alone and the fuel is only mixed by being injected into the combustion chamber separately. Model diesel engines are instead a carbureted two-stroke using the crankcase for compression. The carburetor supplies a mixture of fuel and air into the engine, with the proportions kept fairly constant and their total volume throttled to control the engine power. Apart from sharing the diesel's use of compression ignition, their construction has more in common with a small two-stroke motorcycle or lawnmower engine. In addition to this, model diesels have variable compression ratios. This variable compression is achieved by a "contra-piston," at the top of the cylinder, which can be adjusted by a screwed "T-bar". The swept volume of the engine remains the same, but as the volume of the combustion chamber at top dead centre is changed by adjusting the contra-piston, the compression ratio (swept volume + combustion chamber / combustion chamber) changes accordingly.

Model diesels are found to produce more torque than glow engines of the same displacement, and are thought to get better fuel efficiency, because the same power is produced at a lower rpm, and in a smaller displacement engine. However, the specific power may not be significantly superior to a glow engine, due to the heavier construction needed to assure that the engine can withstand the much higher compression ratio, sometimes reaching 30:1. Diesels also run significantly quieter, due to the more rapid combustion, unlike two-stroke glow engines, in which combustion may still be occurring when the exhaust ports are uncovered, causing a significant amount of noise.

Recent developments in model engineering have produced true diesel model engines, with a traditional injector and injector pump, and these engines operate in the same way as a large diesel engine.