Direct injection means injecting the fuel directly into the cylinder instead of premixing it with air in separate intake ports. That allows for controlling combustion and emissions more precisely, but demands advanced engine management technologies. Unlike petrol engines, diesel engines don’t need ignition system. Due to the inherent property of diesel, combustion will be automatically effective under a certain pressure and temperature combination during the compression phase of Otto cycle. Normally this requires a high compression ratio around 22 : 1 for normally aspirated engines.
A strong thus heavy block and head is required to cope with the pressure. Therefore diesel engines are always much heavier than petrol equivalent. The lack of ignition system simplifies repair and maintenance, the absence of throttle also help. The output of a diesel engine is controlled simply by the amount of fuel injected. This makes the injection system very decisive to fuel economy. Even without direct injection, diesel inherently delivers superior fuel economy because of leaner mixture of fuel and air. Unlike petrol, it can combust under very lean mixture.
This inevitably reduces power output but under light load or partial load where power is not much an important consideration, its superior fuel economy shines. Another explanation for the inferior power output is the extra high compression ratio. On one hand the high pressure and the heavy pistons prevent it from revving as high as petrol engine (most diesel engine deliver peak power at lower than 4500 rpm. ), on the other hand the long stroke dimension required by high compression ratio favors torque instead of power. This is why diesel engines always low on power but strong on torque.
To solve this problem, diesel makers prefer to add turbocharger. It is a device to input extra air into the cylinder while intake to boost up the power output of the engine. Turbocharger’s top end power suits the torque curve of diesel very much, unlike petrol. Therefore turbocharged diesel engines output similar power to a petrol engine with similar capacity, while delivering superior low end torque and fuel economy.