It is known that fuel distribution in a multi-cylinder gasoline engine affects power, fuel consumption, exhaust emissions and vibration of engine.
One accepted method of improving the fuel distribution was to vaporize fuel by heating intake manifold. However, it accompanies with the reduced maximum power.
The effects of riser length, its inner surface shape and exhaust gas recirculation passage on fuel distribution was investigated by actual engine tests.
It is understood that the only acceptable technique to measure fuel distribution among cylinders is the exhaust gas analysis for each cylinder. The measurement of total carbon mole percent of exhaust gas was found to be one of the most convenient and consistent way to quantify the fuel distribution. This was done in this study by the measurement of the combined contributions from CO, $CO_2$ and unburned HC.
The saw-tooth-shape riser produced slightly improved engine performances such as fuel distribution, power and fuel economy.