2 types of Hybrids ... which is more efficient? - Fuelly Forums

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Old 03-27-2008, 07:42 AM   #1
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Exclamation 2 types of Hybrids ... which is more efficient?

1- A gas engine with a supplemental electic engine (like all hybrids today).

2- An all electric car with a small gas or diesel generator to recharge the batteries.

Which is more efficient and why?

What I think;
#2, this is because an electric engine is more efficient, so it would run the car all the time making it more efficient. Also, a generator is always running at its peak efficiency and you only need a small (probably 3-10hp) engine to make enough electricity to recharge the batteries.
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Old 03-27-2008, 08:29 AM   #2
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The generator and motor are only going to be 75% efficient, so with a typical 25% efficiency for the IC engine, you lose 25% in generation and 25% in conversion to motive power such that only about 14% of the energy in the fuel actually makes it to the drivetrain, which typically cuts it by another 15%.... Plus the vehicle has to carry the weight of a motor and a generator...

The other type, about as much energy gets to the road as with a conventional vehicle, with the bonus that when energy is wasted, it reclaims it, at low efficiencies admittedly, but getting 50% of the wasted energy back is a net gain, compared to throwing out 50% of it in the first place....
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Old 03-27-2008, 08:31 AM   #3
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Quote:
Originally Posted by trautotuning View Post
1- A gas engine with a supplemental electic engine (like all hybrids today).

2- An all electric car with a small gas or diesel generator to recharge the batteries.

Which is more efficient and why?

What I think;
#2, this is because an electric engine is more efficient, so it would run the car all the time making it more efficient. Also, a generator is always running at its peak efficiency and you only need a small (probably 3-10hp) engine to make enough electricity to recharge the batteries.
I'm no expert of hybrid electric vehicles, and I'm sure more knowledgeable posters will come in to offer their opinions.

The type-2 is usually called series hybrid. Its main advantage is that the ICE (not the generator) can work at a narrow rpm/load band that offers the maximum efficiency. AFAIK, there's no production consumer cars that uses series drivetrain yet. There are probably some applications on heavy duty and military vehicles. Also, a lot of diesel locomotives are series diesel-electric series hybrid configuration.

The type-1 you mentioned could be a parallel hybrid, like the Civic Hybrid, or a parallel-series hybrid, like the Prius.

IMHO, the parallel-series hybrid probably provides the optimal solution when efficiency/weight/volume/performance/... are considered. It operates as a series hybrid during light-duty cruising to provide good efficiency, and operates as a parallel hybrid when more power is needed to provide good acceleration and gradability. If only fuel efficiency is considered, I think a plug-in hybrid with a series drivetrain probably is the best.
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Old 03-27-2008, 08:43 AM   #4
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I would say 1.

Everything being equal, size and type of gas engine being the same. It's always going to be more efficient to run the wheels directly off the engine because of your conversion losses.

Going from mechanical to electrical ac, converting to dc and charging a battery, then back to ac for the electric motor will lose more than most transmissions. You typically to lose at least 10% at each of those conversions.

Can a 10hp generator keep up with the energy demands?

It is possible that the car could be more efficient though as I'm merely speculating. Lets pick a car, find some inverter and battery specs, choose an electric motor and do the numbers. I'm at work or I'd start.

Quote:
Also, a lot of diesel locomotives are series diesel-electric series hybrid configuration.
There is a reason for that. Those engines hardly ever rev past 1000 rpm, they make around 1250HP around 850 rpm(7720Ft-Lbs of torque at 850), and in order to get an engine like that to pull up to speed you'd need close to 40 gears and a 40 speed transmission that had to handle power numbers like that would be about the size of a house.
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Old 03-27-2008, 09:08 AM   #5
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I think it gets to be "worth it" when you get the IC motor more than 50% efficient in a narrow band, for instance with heat regeneration diesel and gas turbine power plant installations can have overall efficiencies of 75-80% but they can throw a lot more equipment at it, in terms of huge heat exchangers and bottoming cycle turbines, i.e. you could use the same kit to build a 80% efficient hybrid semi-truck... but it wouldn't have a useful load.
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Old 03-27-2008, 11:20 AM   #6
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The Volvo ECC used a reheated gas turbine as a power source for the hybrid.
The S80, S60 body style (.23 Cd) was born out of this endeavor.
As I remember CarandDriver reported getting 55 mpg at 55 mph on the highway.
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Old 03-27-2008, 05:09 PM   #7
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Locomotives use electric drive because there is simply no mechanical drive that will take the enormous engine power of locotive engines. 4500 HP has become common. The biggest mechanical drives are limited to 1300 HP and cannot take the torsional vibration of railroad service.

Straight AC drive IS very efficient. They now use AC motors a variable frequency drives for speed control.
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Old 03-27-2008, 07:12 PM   #8
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Well since the diesel generator is never running the wheels...losses would seem to be at a minimum. The electric motor would be the only mechanical drive system...the diesel generator is only there for charging purposes along with other systems for regenerative possiblilities. I think #2 could be very efficient.

I still think a Diesel Hybrid ...or choice #3... would be the easiset improvement at this moment.
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Old 03-28-2008, 02:04 AM   #9
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Thanks, dkjones96 and Big_Dave, for clarifying the reason electric drive is used on locomotives. I heard when I was a kid that that's what's used and never gave it too much thought. It makes sense that the complexity/availability of a mechanical transmission that satisfies the requirement of a locomotive makes the hybrid drive a more feasible solution.

I maintain my opinion that series hybrid-electric drivetrain probably has more efficiency potential than series (Civic Hybrid) or series-parallel (Prius) drivetrains..

Like Big_Dave said, AC drive can be very efficient. Brushless DC motor drive, including the power converter and the permanent magnet synchronous motor itself, can achieve an efficiency of over 90% easily. Plus the efficiency of the generator, which requires less electronic control and can be optimized for a narrower rpm/power band, the efficiency of the electric system can be easily made to be within mid- to high-80%. This is comparable or a little (~5%) lower than the efficiency of the mechanical transmission in a series hybrid, which is usually around 90%, and probably the same as the electrical portion of the series-hybrid drive.

On the other hand, the ICE in a series hybrid can be made to work at the most efficient rpm/power band, whereas the ICEs in parallel or series-parallel hybrids all deviate from their most efficient operating condition because the ICEs are mechanically connected to the wheels and have to meet the demand at different wheel speeds. The figure below shows the fuel consumption map of the VW 1.9L/90hp TDI engine. It shows how many kg of fuel is used for a kW-hr of energy output at different rpm and power output. The lower the number, the better the efficiency. On a parallel or series-parallel hybrid, the ICE revs up and down, and the throttle position varies a lot, especially in city driving. As a result, it may be consuming 0.22kg, 0.3kg, or even 1kg of fuel for the same 1kW-hr energy output at any moment. Considering this, an average of 0.24kg/kW-hr may be a very optimistic estimation for the ICE in these hybrids. On the other hand, the ICE in a series hybrid can be always working at 0.22kg/kW-hr when it's on. That's a 10% advantage for the series hybrid.



Let's look at the overall efficiency, including the ICE and the transmission/electrical drive. The parallel hybrid has a more efficient transmission by 0-5%. The series hybrid has a more efficient ICE by at least 10%. This makes the series hybrid the most efficient among the three configurations.
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