The recently revealed 2018 KTM 250 EXC TPI and KTM 300 EXC TPI enduro bikes both use a revamped two-stroke incorporating a patented transfer port injection (TPI) system.
The new bike’s motor employs dual fuel injectors mounted inside the transfer port to squirt fuel in at the very last moment before the intake charge reaches the combustion chamber.
With normal carbs or throttle body injection, the air-fuel mixture needs to travel through the crankcase before the downward stroke of the piston pushes it up, via a transfer port, to the combustion chamber. That means it is vital to mix oil with the fuel so that it can act as a lubricant for the crankshaft.
KTM’s system eliminates this. Now there’s just air travelling through the crankcase and the fuel is added later.
The second issue with two-stroke engines is that the exhaust port is open for a long time, and the incoming charge of fresh air and fuel forces the last of the spent gasses out of the cylinder. That means some unburnt fuel escapes before the piston comes up enough to close the exhaust port.
With the KTM system, the fuel is injected a little later, so the initial blast of air driving the exhaust gasses out would have little or no fuel mixed with it, improving the engine’s emissions performance.
By Ben Purvis
