In an attempt to abate soot emissions, diesel fuel for a direct-injection, single-cylinder diesel engine was step-by-step replaced with natural gas. The gas was introduced into the intake system.As a result of this, soot emissions were decidedly improved and maximum power was increased. As an undesirable secondary effect, especially at small loads, there was an unacceptable increase in brake specific gas-diesel fuel consumption and an increase in emission of harmful gaseous exhaust components.Exhaust gas recirculation and throttling were investigated as possible countermeasures. Both are capable of reducing the emission of poisonous gaseous exhaust components at high gas rates, without essentially increasing soot emission. Also, higher part-load consumption could be reduced by these countermeasures.