Some radials (and few V-12s) used extra mixture as a both a charge cooler and an internal engine coolant. Any picture showing a piston plane taking off with black smoky exhaust probably means they are using excess fuel as a coolant. They sure aren't burning it.
A number of American engines, R-2800s in particular, used significantly less fuel when the water injection was activated. They had de-enrichment circuits added to the carb or injection unit.
The alcohol is mainly to keep the water from freezing. Water soaks up a huge amount of heat when it flashes to steam, much more than the same amount (weight) of gasoline.
Hello Shortround6,
Another great example of this is the C3-Einspritzung system that was installed in late model FW 190A/F/G aircraft.
It sprayed fuel directly into one side of the supercharger as an anti-detonant. Apparently the older MW50 system tended to eat spark plugs and this system made for simpler plumbing with a fuel tank replacing the water-methanol tank.
Unfortunately, it seriously raised fuel consumption, but such is the trade off.
as far as exhaust scavenging goes on supercharged engines, look at the difference between the incoming air/mixture pressure and the outgoing.
On an unsupercharged engine the difference between the pressure in the exhaust ports/pipes and the intake manifold is slight, in fact the pressure before the exhaust valve opens might even be higher (most unsupercharged engines operating at under 14.7lbs in the intake manifold) and a lot of the movement of the gases in under momentum?
I believe there is a lot more to the story that you are neglecting here.
I am pretty sure nothing I am describing is new to you but some others may not be so familiar, so apologies in advance.
As you point out: On an unsupercharged engine, the intake manifold is usually running UNDER ambient pressure while the exhaust manifold which gets input from the heated exhaust gases is running ABOVE ambient pressure. So what is keeping the flow moving in the right direction? Momentum is part of it, but something had to develop that momentum to begin with.
The answer is actually the mechanical pumping action of each piston / cylinder.
On the intake stroke, charge moves into the cylinder because the piston is moving down and the volume of the cylinder is increasing and ONLY (mostly true) the intake valve is open.
On the exhaust stroke, combustion gas moves out because the piston is moving up and reducing the space in the cylinder and the exhaust valve is open. Note that the combustion gas is typically quite hot, so the pressure inside the cylinder is likely to be a LOT higher than the exhaust manifold.
Note that although intake and exhaust valves flow the same mass of gas, the exhaust valves are a lot smaller because pressure is higher and they don't need to be as big to flow the same.
The characteristic that a lot of folks may not see is that the flow through each cylinder is not constant; It is a sequence of pulses spaced apart by 720 degrees of engine rotation (for a 4 stroke engine).
This is where momentum of the fuel-air mixture and exhaust gases come into play.
Although the flow in each intake runner / exhaust header is a series of pulses, the flow in the intake plenum / exhaust collector can be much more constant.
If sized properly, the stopping of a column of air as one intake valve closes can be timed to coincide with the opening of the next intake valve and the momentum of the charge air in the plenum "rams" additional charge into the next cylinder.
The same applies to the exhaust header primaries; The closing of one exhaust valve coincides (at the collector) with the opening of the next exhaust valve to produce a scavenging effect.
This momentum idea is often carried a step further with high performance engines.
The spent charge inside the cylinder also has a momentum as it leaves the cylinder.
If the Intake Valve is opened slightly before the Exhaust Valve is closed, this momentum can be used to pull more charge air into the cylinder.
This characteristic is called Valve Overlap. If designed properly, at high RPM, an engine is sometimes (not often) capable of actually pumping more air through than its swept volume would indicate. Volumetric efficiency can be above 100%.
The cost is that valve overlap reduces the efficiency of the mechanical pumping motion of the engine at low RPM.
This degrades the signal to the carburetor / MAP sensor and low RPM operation and off idle response is sacrificed.
This is why some high performance engines are often set to idle fairly high and miss a lot at low RPM
This is my understanding of some of the factors involved.
- Ivan.
Its second speed power is harder to compare as heights are different due to different supercharger speed gearing.
