One has got into the habit of mentioning direct- petrol and direct Diesel injection in one breath. On this page, we would like to elaborate the differences very clearly. We'll do this using an aviation engine as an example, in fact, one that is suspended from the ceiling in the Daimler Museum in Stuttgart, the reason perhaps, why so little notice is taken of it.
The above mentioned is a super-charged V12-aircraft engine (production start in 1937). Depending on model and the air traffic demands, it had a good 30 liters of cubic capacity and 736 kW (1000 Hp). Another small feature: It's two cylinder banks, through an ingenious interlocking of the opposing connecting rods, were not offset against each other.
So much for the upside-down engine, which functioned not much differently than it did previously as a carburettor engine, except that it had a lower performance, particularly when operating in great heights. What we're interested in, is the petrol injection pump, which at first sight is not unlike those found in Diesel engines.
Of course, the fuel-air mixture and ignition in the Diesel engine are achieved completely differently. It is injected under extremely high pressure and already then, does the combustion begin. Not much time to form the mixture. In this aircraft engine it's completely different. In this case, already during the intake-stroke, the fuel is injected directly into the combustion chamber.
Because a vacuum still prevails here, in theory, the injection pressure chosen can be as low as you like. However, to achieve a good atomisation, a double-figure pressure value (in bar) is necessary. A very important fact that should be mentioned, is that the components of the injection system can thus, be produced much more cheaply. The mixture formation time is also much longer than in a diesel engine, because there's plenty of time until the, as before electric ignition, takes place.
Should you be interested in this injection regulation, you can study the quite similar Diesel-system here. The important differences are the double controlling-edge and the additional lubrication of the pump-piston with engine oil.
Apart from that, by no means can virtually any amount of petrol be injected, The fuel amount must always correspond to the amount of intake-air. Have a look at the complicated barometric regulation. At the bottom of the page a detailed description of the petrol-injection pump from a similarly built Jumo-engine can be downloaded. This engine was used in Messerschmitt-, Heinkel- and Junkers-aircraft. 11/13