Diesel Hybrids
The hybrid trains and busses described above are diesel powered – as are nearly all commercial vehicles. This is simply because compression ignition (diesel) engines are the most efficient internal combustion engines available, and also have an enviable reliability record. Diesel hybrid delivery vans are now appearing as well.
Since diesels are much more efficient (40-45% thermal efficiency) than gasoline engines (25-30%), marrying them to hybrid drive trains should be very attractive. In fact, it is not.
The first section described how spark ignition engines are highly inefficient when operated at low power output. This is not true of diesels. A diesel operating at its peak efficiency uses about 35% less fuel than a similar gasoline engine operating at its peak efficiency. However, while a gasoline engine at 10% of its power output uses 3 times as much fuel to do a specific amount of work, a diesel running at low power uses just 20% more fuel than at power. One reason for this is that diesels do not throttle their air intake, so pumping losses are low and do not increase at low power.
This means that diesel engines benefit less from operating through a hybrid drive train. In fact, to make a diesel hybrid drive train cost effective, the vehicle really must operate almost exclusively in urban traffic. However, diesels tend to be noisier than spark ignition engines, so large vehicles like busses and delivery trucks become more “city friendly” if equipped with a hybrid drive train that allows them to run silently (electrically) at least part of the time.
Several parcel delivery fleets run trials with diesel hybrid vans and mid-weight trucks. Driving exclusively in city traffic, and fully warmed up, they anticipated 30-40% improvement in fuel economy. Part of this must be due to the heavy fuel consumption penalty of the automatic transmission these vehicles always have. One test showed that the hybrid drive improved fuel economy by 27% over identical trucks with manual transmissions but by a massive 59% over identical trucks with automatic transmissions! This suggests that, driving in urban traffic, a heavy truck automatic transmission consumes around 30% more fuel than the same truck with a manual gearbox! The large penalty is due to the fact that the torque converter is always “operating” in traffic; it never has the opportunity to lock out its fluid drive.
The hybrid drive is usually mated to an “automated manual” transmission which replaces the power- and fuel-wasting torque converter (converts 15-20% of engine power to heat in urban driving) with an “old-fashioned” friction plate clutch. The clutch and gearbox are automatically operated by a computer.