Quote:
Originally Posted by alohaspirit
. . .
So why is there no diesel Toyota Prius yet?
. . .
|
The Prius uses an Atkinson cycle engine:
- 8-to-1 compression stroke - this is accomplished by leaving the intake valve open during nearly half of the compression stroke. Part of the charge is pushed back into the intake manifold but be drawn in by another cylinder. This also reduces the amount of energy needed to turn over the engine and reduces internal, power robbing, friction. In contrast, the diesel requires a very strong motor to overcome the 20-to-1 compression stroke.
- 13-to-1 expansion stroke - the exhaust stroke provides a ratio that allows a very high ratio of energy extraction. A typical diesel is 20-to-1 so the Prius brake specific fuel consumption approaches diesel with the advantage of a significant reduction in NO{x} production.
- avoiding low-power, inefficient operation - the electric part of the Prius allows the engine to run at a higher power but very efficient range than needed at slow speeds and it stores the extra energy in the battery. Then the engine shuts down and the stored electrical energy powers the car without burning any more fuel. This advanced transmission is what is missing from diesel vehicles which at lower power settings has to always deal with engine friction losses, a lower BSFC than the Atkinson engine.
- saving brake energy - another part of the Prius is saving braking energy as battery charge for use later. This is another part missing from diesel vehicles.
What it comes down to is the diesel doesn't offer enough to compensate for its short-comings: (1) higher NO{x} emissions, (2) inefficient at low power settings, (3) absence of regenerative braking, and (4) difficulty handling start-stop engine operation. They also tend to be heavy.
It is likely that as combustion temperatures are mastered, some future engine may support super-lean yet NO{x} safe systems. Perhaps some clever combination of cooled exhaust temperature and direct fuel injection but right now, these are discussed in university labs, not really in production vehicles ... except for the 2010 Prius.
At high power settings, the 1.8L Prius engine cools part of the exhaust and feeds it into the intake manifold. This reduces the peak combustion temperature so the Prius BSFC remains high even climbing a hill at high speed. The mixture remains lean but the engine doesn't burn out the catalytic converter or overload it with NO{x} products.
The Prius also uses fuel injection but not to direct combustion. Instead, the fuel is carefully metered and there is time to achieve an optimum fuel-air mix for combustion. In contrast, the diesel is always 'lean burn', it has to be, and that leads to NO{x} and other problems. It is the Atkinson cycle, the reduced fuel charge at low power settings that gives it the "on the road" advantage.
Bob Wilson