Why Superchargers Don't Work
There has been lots of talk on these for a long time on many RC forums, so I thought I’d create this just to clear up the myths.
Engines have an ideal ratio of fuel and air, the right "mixture". This is why we tune the engine, to make this balance perfect. Therefore we can't just increase the amount of fuel to make the engine go faster, as this would just make the mixture too rich and slow the engine down. Consequently the only way of getting a faster engine once tuned, is to get both more fuel and air into the engine. This is exactly what a supercharger is designed to do. They compress fuel and air, so you can fit more "mixture" into the same space. More mixture means a bigger bang - more power.
Superchargers are very effective on both 2 and 4 stroke engines. Without going into technical detail about how they work, the basic principle is easy to understand. Superchargers are like giant fans connected to the crankshaft for power. As the engine turns over the ‘fan’ spins and forces air into the engine. The force on the fuel/air mixture is large and thus compresses the mixture, so it squeezes both more air and fuel into the same size space compared to an engine without one. This means you can force more fuel and air into the engine, so there is a larger bang and greater power!
Wow, nice and simple really isn’t it? So, why doesn’t it work with the engines we use in RC cars? The answers are numerous, but the largest factor is that the ports of our RC engines are ‘inline’ and do not use a valve system.
This should help to explain:
As you can see the intake and exhaust port on the radio control engine are around the same height, they are roughly inline with each other. So forcing this high pressure fuel and gas mixture through the engine won’t do anything. The faster the fan pushes the mixture, it isn’t going to compress. It will just pass straight in the intake port, over the piston and out through the exhaust port. In other words, you’ll be wasting fuel as it’ll just push this un-burnt mixture out into the exhaust.
Some two stroke engines, not used in RC, have valves. With these when the intake port is open, the
exhaust port is closed. This allows the giant ‘fan’ on the supercharger
to force more of the fuel/air mixture into the engine creating the bigger
‘bang’ and greater power. Also another method used in other engines, not RC, to have the exhaust port lower then the intake port. This
way when the piston is at the bottom the mixture can be forced and compressed
as both ports are not open at the same time.
However, an RC engine is neither of these so a supercharger is effectively useless on an RC engine. One justification I have heard is that the input port on the crankshaft is not always open when the ports are. This is true and can clearly be seen from the cut-away image above. This does allow the air to compress before it reaches the crank-shaft. However the instant the it passes through the crankshaft, the system is open again and the pressure will equilibrate to that of the exhaust port.
Yet another justification I have heard about RC engines using superchargers is there is always an on demand supply of the fuel/air mixture entering the engine. In my opinion, this is nonsense. RC engines draw in the mixture, and any good engine is designed to draw the optimum amount. A supercharger regulates the flow, which I can only see as a bad thing. This is needlessly wasting fuel, and could even limit the mixture and decrease performance if the supercharger isn’t set-up to provide the demand to the engine throughout the rev range.
Another negative of these model superchargers is the efficiency loss on the engine. The supercharger is powered directly by the engine output. 2-stroke engines, particularly our model engines, are very high revving. The crankshaft can rotate at speeds in excess of 35,000rpm on some engines, the actually torque produced is quite low and power is obtained by gearing the engine down. At low revs in particular there is very little torque in model engines, therefore having a supercharger demanding power from the engine, means less power to the wheels. This could cause an adverse affect on the engine, and actually reduce low end power. Through-out the power band the supercharger will create friction and reduce efficiency as a consequence.
Furthermore a supercharger will decrease performance by adding weight. Suddenly a seemingly good way to get power turns into a bit of a nightmare! If you want a powerful car, firstly buy a good engine, and very importantly a good tuned pipe. A good quality tuned pipe will give you 10-20% more power then a poor one. Finally, the easiest way to get engine power is to learn how to properly tune your engine. It can take years to masters, and people can still get it wrong. If that's the case, copy me and get help from a pro!