Carburettor or Fuel Injection, the good and bad


Fuel injection is a developing technology, not a mature one. When FI first appeared on large numbers of bikes, there were problems. In one class of problems, the fuel sprays failed to evaporate fully, droplets hit the cool cylinder walls, and fuel was swept down into the crankcase by taper-faced, 'down-passing' piston rings. Each time the rider checked the oil level, it was higher than the time before!

One solution to this problem has been the adoption of dual throttle butterfly systems. By analogy with the long-serving constant-vacuum (CV) carburettor, the rider controls one butterfly but the opening of the second is delayed to allow fuel delivery to keep up with air delivery. In tyhe CV carburettor, a vacuum diaphragm controls the rate of opening of the second butterfly, and in the case of dual-butterfly injection systems it is the engine's computer that controls it.All this is necessary because liquid fuel is 600 times heavier by volume than air. When the rider snaps the throttle, the engine's pistons send their invitation to the intake air at the speed of sound, but the heavier fuel will get left behind unless some system evens out the race. When the air gets there first, hesitation results.

Through years of dismal ignorance I assumed that because fuel injection operates at 4 to 6 atmospheres of fuel pressure, it must do a better job of fuel droplet break-up than a carburettor does. In the early days of FI this was emphatically not the case! A carburettor bleeds air into its fuel stream between the main jet and its delivery into the intake air stream. This not only enlivens fuel delivery by reducing its density, it also helps in droplet breakup. At part throttle, fuel droplets must also pass through the restriction between throttle and carburettor bore where velocity is high.

High air velocity is converted to pressure upon hitting the upstream side of a fuel droplet. That pressure flattens and then punches in the face of the fuel droplet, whose surface tension then pulls it first into a donut-like ring, and then breaks up the ring into a circlet of tiny sub-droplets.

Even at full throttle, a carburettor has restricted throat area to produce high airflow velocity. Velocity generates the partial vacuum that pulls fuel in from the float bowl.

A big initial attraction of fuel injection was that it needed no such restriction, or venturi, because its fuel was supplied under pump pressure. Thus, the engine's intake could be made as large as necessary for maximum flow. This increased airflow was supposed to translate directly into power.

Unfortunately, when the fuel droplets spray out into this more leisurely flow, there is insufficient velocity to break them into a fine spray. The result is that largish droplets, of the order of 50 to 150 microns diameter, are what the engine receives. As large as they are, such droplets have a lot of volume in relation to their surface area, for volume increases as the cube of diameter, while surface area increases only as the square. This makes them slower to evaporate than are smaller droplets, with the result that they may splatter against the cylinder walls to form a fuel film that gets scraped down to the crankcase by the piston rings.

Japanese industry responded with the multi-hole injector, which has 6 to 12 tiny holes rather than a single spray orifice.

Sprays form droplets as emerging streams of fuel become unstable, begin to wiggle, and rapidly break up into fragments that pull themselves into droplets. The smaller the stream, the smaller the resulting droplets. This was a wonderful advance because under most conditions it produced a fully evaporated fuel vapour. The hardest condition of all is full throttle, when there is very little time between injection of the fuel under the throttle butterfly and its arrival in the cylinder. To extend the evaporation time, a second injector is used, for there is no room below the throttle butterfly for a conventionally located injector.

In dual-injector engines, as rpm rises the computer delivers more fuel through the showerhead and less through low-speed injector under the butterfly. Ducati had a hard job achieving rapid throttle response with its single showerhead injector, but they learned a few tricks in the process. Now FI technology may be ready to move on again, with a concept called GDI, for Gasoline Direct Injection.

Direct cylinder gasoline fuel injection has existed for a long time, German aircraft engines used it throughout WWII, but GDI goes beyond that. First, a GDI injector is very like the special injector developed for ultra-low-emissions two-strokes in that it rapidly generates extremely small droplet sizes that evaporate promptly. Second, the GDI concept can operate in an extremely lean, economical stratified-charge mode.
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