Why no diesel Prius?

[Arragonis]

I know its an older post in this thread but :

"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."

Why don't / can't diesels do this ?

[Christ]

Because it requires a complex braking system that converts braking friction to electrical energy, storing it in capacitor or battery banks for future use, thus including an electric motor drive which can properly utilize the electrical energy.

Braking could, however, be used to charge the accessory battery, especially on non-computer controlled systems. Enough people use the brakes enough times that charging the battery would be a menial task, I would assume. It would safe efficiency of the engine, as well, since it wouldn't be driving an alternator.

[Arragonis]

But wouldn't a Diesel Prius just be a Prius with a Diesel instead of a Petrol engine ? Therefore it would have anything a Prius has, plus added Diesel rattly goodness ?

[bwilson4web]

The real problem is the absence of a systems analysis of a hybrid vehicle. The diesel is being treated as "blessed" or a "magic technology" without thinking about the whole vehicle profile. In fact, it is the blessing of a particular engine technology and failure to do a complete systems analysis that spells doom for many of the faux hybrids announced and coming on market.

There is a very simple relationship, the ratio of traction power to ICE power that tells whether or not a particular vehicle will be a successful hybrid or a 'greenwash' hybrid. Understand, I have no problem with manufacturer's trying to sell 'greenwash' hybrids but I won't put my money in them.

Bob Wilson

[alohaspirit]

an engine shut off feature will be included in the new 4 door porsche

claimed at increasing fuel economy by 10%

this is what is already in many hybrids around the world


hopefully it can be incorporated in all new cars

(or a cheap add on kit)

[Christ]

Originally Posted by Arragonis

But wouldn't a Diesel Prius just be a Prius with a Diesel instead of a Petrol engine ? Therefore it would have anything a Prius has, plus added Diesel rattly goodness ?

That's the way I figure it, anyway. I figure that one would just replace the gas engine in the Prius with it's diesel counterpart, except there would need to be a complex monitoring system involved that would allow for the features currently used in the Prius, so the diesel would have to have an electronic feedback system, like the newer VW TDI motors. (Hey, there's a thought...)

[roflwaffle]

Originally Posted by Christ

Of course, if a smaller, lighter, more aerodynamic car is making nearly the same HP as a much larger/heavier truck, it will make more particulate emissions because the engine won't be loaded sufficiently to be efficient, and will be burning fuel uselessly. Does that make sense?

From the perspective of BSFC it's consistent, but emissions tend to be the opposite. At least for older diesels, particulates go up w/ load, NOx/CO goes up w/ load and speed, and HCs are roughly the same.

Originally Posted by Christ

Seems like taking some of the power away from the car, or chaning the power curve itself to allow for more load at lower RPM, while still allowing the power to be "fun" when the operator requests it is in order?

That's pretty much what manufacturers have been moving towards for the past decade or so via drive-by-wire, variable cam phasing, eleventy billion speed automatic transmissions, cooled EGR, and so on.

[Arragonis]

Originally Posted by Christ

That's the way I figure it, anyway. I figure that one would just replace the gas engine in the Prius with it's diesel counterpart, except there would need to be a complex monitoring system involved that would allow for the features currently used in the Prius, so the diesel would have to have an electronic feedback system, like the newer VW TDI motors. (Hey, there's a thought...)

Yep, thats what I thought. Just like Honda, Toyota make Diesels for Europe (D4-D - although the one that used to be in the Aygo was from PSA) which are pretty good so its just really a fitting thing. Here in the UK we have Corolla (or whatever the new one is called) Diesels as well as the Prius and the MPG differential is favouring the conventional car.

BTW (read the thread again) - I believe that Diesels have a lower redline because Diesel burns slower than petrol, hence also the longer stroke normally found in Diesel motors.

As for smoothness, a dual mass flywheel (DMF) smooths out the stronger pulses too, so it could be made to work IMHO.

[Christ]

Originally Posted by roflwaffle

From the perspective of BSFC it's consistent, but emissions tend to be the opposite. At least for older diesels, particulates go up w/ load, NOx/CO goes up w/ load and speed, and HCs are roughly the same.
That's pretty much what manufacturers have been moving towards for the past decade or so via drive-by-wire, variable cam phasing, eleventy billion speed automatic transmissions, cooled EGR, and so on.

Thanks for the link to the paper. Saved that one.

Unfortunately, the maps shown in that paper reference an increase in emissions when road speed and load are increased steadily, which should be obvious.

What I'm referring to is steady state RPM with varying loads. In other words, when you're cruising, and you start going up a hill, then going down the other side. If you're not using all the available power, you're pretty much wasting fuel. You can tell if you're properly loading your diesel engine by checking EGT's though. You should be in the ~600* range for best efficiency in most cases, IIRC.

The thing about diesels is that it's hard to control load without metering airflow, so it would be very difficult to test for emissions properly. When you floor it in a diesel, it starts injecting fuel based on a pre-set pump that was supposedly tuned.

The general rule of thumb is to turn it up until there's smoke, then turn it back down until just after the smoke disappears. Of course, if you're interested in better fuel economy than power, you could back it off a bit more, as well.

Often, the fuel rack limiter can be adjusted to not allow full throttle fuel input, as well, further reducing max power output by a negligible amount, and reducing the chances of smoke under harsh conditions.

[Christ]

Originally Posted by Arragonis

Yep, thats what I thought. Just like Honda, Toyota make Diesels for Europe (D4-D - although the one that used to be in the Aygo was from PSA) which are pretty good so its just really a fitting thing. Here in the UK we have Corolla (or whatever the new one is called) Diesels as well as the Prius and the MPG differential is favouring the conventional car.

BTW (read the thread again) - I believe that Diesels have a lower redline because Diesel burns slower than petrol, hence also the longer stroke normally found in Diesel motors.

As for smoothness, a dual mass flywheel (DMF) smooths out the stronger pulses too, so it could be made to work IMHO.

Diesels don't always have a lower redline. It's a matter of how they're produced. Diesel engines inject fuel as the combustion stroke is going on, from just after TDC (technically at or before, but bare with me) until some point about 50% of combustion stroke, or ~45* of crank rotation in a 4 cylinder.

Long stroke engines, gas or diesel, will have a higher torque rating than their short-stroke counterparts. The higher compression engines are capable of extracting more power from the fuel than lower compression engines, and tend to burn cleaner as well. Shorter stroke engines have the advantage of higher RPM, and thus higher HP figures.

Long stroke engines inherently don't rev extremely high, as the rod/stroke ratio wouldn't allow for it without damaging parts eventually. Also, the parts in diesel engines tend to be very heavy compared to those in gas engines, and more rotating mass means more inertia, which means more stress on parts, which means lower redline.

I hope that helps.