Any reason semis do not use diesel-electric engines?

223rem wrote:

> Harry K wrote:
> > 223rem > wrote:
> >> ?
> >
> > Errm...fuel economy? Every time you convert energy from one form to
> > another you lose energy in the conversion.
>
> Yes but the diesel engine driving the electrical generator runs at a
> constant, most efficient RPM, as opposed to a diesel engine running the
> transmission directly. That ought to more than compensate for the losses
> in converting mechanical work into electricity.

That's because trains can use that 'constant power' effectively. Road vehicles
can't.


> Besides, isnt this the principle behind "hybrid" vehicles?

Not exactly. It does help improve the efficiency of the ICE part of their
operation though.

Graham

223rem > wrote:
>Harry K wrote:
>> On Jun 6, 5:44 pm, 223rem > wrote:
>>> ?
>>
>> Errm...fuel economy? Every time you convert energy from one form to
>> another you lose energy in the conversion.
>
>Yes but the diesel engine driving the electrical generator runs at a
>constant, most efficient RPM, as opposed to a diesel engine running the
>transmission directly. That ought to more than compensate for the losses
>in converting mechanical work into electricity.

In a conventional diesel-electric system, the electric motor and generator
are basically acting as a transmission to trade torque for speed or vice
versa.

Conventional trucks, though, spend most of their time at highway speed, so
the efficiency is not improved by going diesel-electric. And they also
employ transmissions with a godawful number of gears to allow them to get
efficiently up to highway speed.

>Besides, isnt this the principle behind "hybrid" vehicles?

A hybrid vehicle also has a battery bank, allowing it to produce peak power
that is much greater than the peak power the engine is capable of. This
sort of design IS a serious win for long-haul trucks, and a couple of truck
manufacturers are working on hybrids right now.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On Jun 7, 12:40 am, "Tim B" > wrote:
> "223rem" > wrote in message
>
> . ..
>
> > ?
>
> Hybrid semis are being developed.
>
> http://www.peterbilt.com/index_new_mor.asp?file=2100

New class 8 tractors are expensive as it is now? A APU to start the
engine and run climate control when idling would be very responsible.

On Jun 6, 11:03 pm, 223rem > wrote:
> Harry K wrote:
> > On Jun 6, 5:44 pm, 223rem > wrote:
> >> ?
>
> > Errm...fuel economy? Every time you convert energy from one form to
> > another you lose energy in the conversion.
>
> Yes but the diesel engine driving the electrical generator runs at a
> constant, most efficient RPM, as opposed to a diesel engine running the
> transmission directly. That ought to more than compensate for the losses
> in converting mechanical work into electricity.
>
> Besides, isnt this the principle behind "hybrid" vehicles?

One of the big advantages of the Diesel engine is that part throttle
operation is not nearly as harmful to efficiency as with the spark
ignition engine. Thus, while there would be some efficiency
advantages of a hybrid, they would be far less than for a gasoline
engined vehicle. Therefore the additional weight and cost of the
batteries, the large generator and the electric motor may not have the
economical payoff.

There is little efficiency advantage for a diesel electric per se (not
hybrid).

The reason diesel electric locomotives got their start is that the
clutch and transmission for high powered systems was difficult in the
days when diesel locomotives were just getting started. But GE had
already been making large electric motors and generators and it was an
easy solution. There WERE some straight diesel-mechanical (and also
gas-mechanical) locomotives but they had lots of problems.

Don Stauffer in Minnesota wrote:

> 223rem > wrote:
> > Harry K wrote:
> > > 223rem > wrote:
> > >> ?
> >
> > > Errm...fuel economy? Every time you convert energy from one form to
> > > another you lose energy in the conversion.
> >
> > Yes but the diesel engine driving the electrical generator runs at a
> > constant, most efficient RPM, as opposed to a diesel engine running the
> > transmission directly. That ought to more than compensate for the losses
> > in converting mechanical work into electricity.
> >
> > Besides, isnt this the principle behind "hybrid" vehicles?
>
> One of the big advantages of the Diesel engine is that part throttle
> operation is not nearly as harmful to efficiency as with the spark
> ignition engine. Thus, while there would be some efficiency
> advantages of a hybrid, they would be far less than for a gasoline
> engined vehicle. Therefore the additional weight and cost of the
> batteries, the large generator and the electric motor may not have the
> economical payoff.
>
> There is little efficiency advantage for a diesel electric per se (not
> hybrid).
>
> The reason diesel electric locomotives got their start is that the
> clutch and transmission for high powered systems was difficult in the
> days when diesel locomotives were just getting started. But GE had
> already been making large electric motors and generators and it was an
> easy solution. There WERE some straight diesel-mechanical (and also
> gas-mechanical) locomotives but they had lots of problems.

There were diesel hydraulics in Europe too.

We used to have some ancient diesel-mechanical commuter trains on the line near
me decades back. They were notorious for 'throwing' a propshaft which might then
puncture a fuel tank. One fire a year was about average. No-one was ever
seriously hurt other than spraining joints / whatever from jumping down to track
level IIRC.

Their automatic gearboxes also gave them a curious acceleration. Their
diesel-hydraulic counterparts that operated alongside them didn't suffer these
problems.

Graham

> A hybrid vehicle also has a battery bank, allowing it to produce peak power
> that is much greater than the peak power the engine is capable of. This
> sort of design IS a serious win for long-haul trucks, and a couple of truck
> manufacturers are working on hybrids right now.

I'd wonder if it's a huge win, rather than just something of a win,
for long-haul, over-the-road trucks (unless they can greatly simplify
the powertrain). I'd like to see the math -- probably best thought
of as a family of curves for different kinds of trips and terrain.
I'd expect that medium and heavy trucks used for local delivery might
be a whole different kettle of efficiency, and a plug hybrid version
might also allow a "zero emissions" variant of those (you can do
things with large vehicles that return to the barn every night that
might not be practical with either OTR rigs or cars).

I've seen a claim of $9k/year of fuel savings for the recently
delivered hybrid Pete, in an unspecified duty cycle in the service of
Wal-Mart. Dunno how that will prove out, nor how broadly applicable
the numbers will be in other usage scenarios, but it's an attractive
idea.

I've been seeing a few hybrid city buses lately, and boy, does that
intuitively make sense, their duty cycle usually being the very
picture of stop-and-go driving. Regenerative braking should help
reduce the rate at which they frit away brake linings too.

--Joe

Don Stauffer in Minnesota wrote:
>
> On Jun 6, 11:03 pm, 223rem > wrote:
> > Harry K wrote:
> > > On Jun 6, 5:44 pm, 223rem > wrote:
> > >> ?
> >
> > > Errm...fuel economy? Every time you convert energy from one form to
> > > another you lose energy in the conversion.
> >
> > Yes but the diesel engine driving the electrical generator runs at a
> > constant, most efficient RPM, as opposed to a diesel engine running the
> > transmission directly. That ought to more than compensate for the losses
> > in converting mechanical work into electricity.
> >
> > Besides, isnt this the principle behind "hybrid" vehicles?
>
> One of the big advantages of the Diesel engine is that part throttle
> operation is not nearly as harmful to efficiency as with the spark
> ignition engine. Thus, while there would be some efficiency
> advantages of a hybrid, they would be far less than for a gasoline
> engined vehicle. Therefore the additional weight and cost of the
> batteries, the large generator and the electric motor may not have the
> economical payoff.
>
> There is little efficiency advantage for a diesel electric per se (not
> hybrid).
>
> The reason diesel electric locomotives got their start is that the
> clutch and transmission for high powered systems was difficult in the
> days when diesel locomotives were just getting started. But GE had
> already been making large electric motors and generators and it was an
> easy solution. There WERE some straight diesel-mechanical (and also
> gas-mechanical) locomotives but they had lots of problems.

The big quarry size dump trucks of current generation are diesel
electric as well.

On Jun 6, 9:03 pm, 223rem > wrote:
> Harry K wrote:
> > On Jun 6, 5:44 pm, 223rem > wrote:
> >> ?
>
> > Errm...fuel economy? Every time you convert energy from one form to
> > another you lose energy in the conversion.
>
> Yes but the diesel engine driving the electrical generator runs at a
> constant, most efficient RPM, as opposed to a diesel engine running the
> transmission directly. That ought to more than compensate for the losses
> in converting mechanical work into electricity.
>
> Besides, isnt this the principle behind "hybrid" vehicles?

I suspect the additional weight and cost of the generator and motor(s)
would more than offset any loss due to varying speeds. They would
have to pack that weight all the time.

Harry K

Long ago, there were some small air powered Locomotives.They were mostly
used in mines.There was even a car company long ago by the name of
Locomobile, but they had gas engines.A couple of buddies of mine are
Railroad nuts, they can talk Railroading with you all day long.I own a
big old Chattanooga Leo # 3 Railroad Depot heater.It is sitting on my
concrete front porch.
cuhulin

Harry K wrote:
> On Jun 6, 9:03 pm, 223rem > wrote:
>> Harry K wrote:
>>> On Jun 6, 5:44 pm, 223rem > wrote:
>>>> ?
>>> Errm...fuel economy? Every time you convert energy from one form to
>>> another you lose energy in the conversion.
>> Yes but the diesel engine driving the electrical generator runs at a
>> constant, most efficient RPM, as opposed to a diesel engine running the
>> transmission directly. That ought to more than compensate for the losses
>> in converting mechanical work into electricity.
>>
>> Besides, isnt this the principle behind "hybrid" vehicles?
>
> I suspect the additional weight and cost of the generator and motor(s)
> would more than offset any loss due to varying speeds. They would
> have to pack that weight all the time.
>

Wouldn't the loss of the transmission offset that? And there is
already a generator (alternator), it just needs to be bigger now.

To recap:
+ smaller more efficient IC engine (diesel or gas, or even a turbine)
+ no transmission
+ regenerative braking
- larger heavier battery (pack)
- larger heavier generator
- electric motors to drive the wheels

It's all a matter of proper compromise, I think. It is workable for some
applications right now, and will be for all, when sufficient advances in
battery technology are made.