Consider the DA-42, it has no mixture control. Is that because it is run on Jet-A/diesel? Or is the mixture control automated?

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  • $\begingroup$ Related $\endgroup$ – Pondlife Jun 13 '17 at 12:24
  • $\begingroup$ @Pondlife, actually no, it isn't related. The Rotax engines are still spark-ignition, just with automatic mixture control, while here we talk about compression-ignition engine, where the principle is completely different. $\endgroup$ – Jan Hudec Jun 13 '17 at 18:34
  • $\begingroup$ @JanHudec I see what you mean. I was looking at it as a very general 'aircraft without mixture controls' topic, but you're right that it isn't a direct comparison. $\endgroup$ – Pondlife Jun 13 '17 at 23:59

For fuel, any fuel, to burn, it must be mixed with suitable amount of air. Neither too little nor too much would burn. Spark-ignition engines pre-mix the fuel with air, so they must do so in the right ratio, hence the need for mixture control (which might be automated, but it must be there).

However, compression-ignition (Diesel) engines are different. They inject fuel directly into the cylinders and as the fuel spreads from the injecting nozzle, it mixes with air and ignites at the moment it is sufficiently mixed. The cylinders always take full charge of air, otherwise the compression would not heat the air enough to ignite the fuel.

At low power, the flame is only small around the nozzle and at the large power it expands to fill the cylinder, but either way there is a point where the mixture is just right and the fuel ignites. Therefore compression-ignition engines don't need mixture control. The power lever simply controls the fuel flow into the engine and there is no throttle on the air intake (so calling power lever ‘throttle’ is inaccurate for Diesels (and turbines)).

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  • $\begingroup$ In your first para: "Compression-ignition engines pre-mix the fuel with air" shouldn't that be "Spark-ignition engines..."? $\endgroup$ – FreeMan Jun 13 '17 at 19:24
  • $\begingroup$ @FreeMan, thanks. Of course I meant to say ‘spark-ignition’ there. $\endgroup$ – Jan Hudec Jun 13 '17 at 19:28
  • $\begingroup$ Assumed so, but you know what happens when you assume... :) $\endgroup$ – FreeMan Jun 13 '17 at 19:37

In a gas engine, the fuel is mixed with air, usually in a carburetor, before entering the cylinders, but there is no such pre-mixing at all in the diesel, where a metered amount of fuel is injected directly into the combustion chamber, in the midst of very hot, highly compressed air. Thus, the diesel does always work with an excess of air.

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  • $\begingroup$ I don't think the diesel is injected into the compressed air. IIRC, it happens earlier in the cycle. This gives the diesel time to disperse. The mixture is then compressed and ignites everywhere. $\endgroup$ – MSalters Jun 15 '17 at 10:52
  • $\begingroup$ @MSalters What you describe is not a Diesel, but an oil engine of the Hornsby-Akroyd type, later known as hot-bulb engines. Sometimes, they are (improperly) called 'semi-diesels'. They were much used in agricultural machinery and in small fishing boats. $\endgroup$ – xxavier Jun 15 '17 at 11:37
  • $\begingroup$ No, the "ignites everywhere" specifically excludes hot bulb engines. Compression ignition (which is characteristic for diesels) is how you achieve that. $\endgroup$ – MSalters Jun 15 '17 at 12:00

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