I have been lurking here for a while trying to learn more about hybrids as I am interested in replacing my '96 Metro with something that has AC.
I am a mechanical engineer and thought I had a good understanding of technical things. Other than regenerative braking, I don't see why hybrids do so much better than non hybrids. I do see an aerodynamic drag benefit of the Insight and the HCH has a smaller ICE than it's non hybrid counterpart. I also recognize that the Prius has a smaller ICE than it would have if it were offered in non hybrid form.
Although it's non on my short list of cars I'm considering, for the sake of my question, let's look at the V6 hybrid and non hybrid. They are almost the exact same car so aerodynamics can be eliminated as a benefit as can engine differences. The EPA estimates for the hybrid and non hybrid are 29/37 and 21/30 respectively. This is an increase of 38% city and 23% highway. Can a car realize a 38% purely from regenerative braking? Additionally, I can't imagine that the hybrid should help much on a road trip where the brakes aren't used much. If fact, I would expect there to be a small 'hybrid penalty' on a road trip due to the increased battery weight.
Thanks for your time,
Kevin
Last edited by Kevin Slattery; 08-22-2005 at 12:58 PM.
Reason: typo
___In the case of the AH vs. the std. V6 Accord, there are 2 large items differentiating the two in terms of FE. One is VCM which is a 3-cylinder mode of operation for the AH vs. the std. V6’s all 6, all the time. Far less fuel consumed per mile traveled when in this mode of operation. Second is Autostop. At light, signs, stops, the ICE shuts down. An idling V6 still gulps fuel unlike a shutdown one. These are the 2 main reasons why the AH has a much higher EPA rating then the non-hybrid V6 in the (2) EPA test cycles of merit.
In addition to regenerative braking, the electric motor will also generate electricity to be stored when you let off of the accelerator while going down a moderate to steep grade. This energy can then be used to power the electric assist as you use more throttle to maintain speed going uphill. This is how they can eek out a few more MPGs even on the highway.
All of the above, plus the civic and insight have 'lean burn' capability when the engine is at steady load. And the Ford/Toyota hybrids use the electric motor when the most gas is burned- starting and accelerating- giving further advantage. The energy is put back in at lower cost over time- braking and engine generation.
For an apples-to-apples comparison of the 3 vs 6 cylinder modes, look at the Odyssey- same engine in all models, 10% better mpg for the ones with VCM. (19/25 vs 20/28). As expected, the hwy benefit is much greater than the city, when the engine is running on all 6 more frequently. There is no autostop, and no IMA on the Odyssey. Purely VCM making the difference.
For those of us with VCM, the name of the game is keeping the car in 'eco' mode as much as possible. I regularly get 43mpg on my 33mi commute, both ways, with the eco light on >90% of the trip. But even a little town driving drops my entire tank into the mid 30s.
A few more small points:
Higher thermodynamic efficiency of ICE (in part because engine is smaller, but also because it runs under high load via the coupled electric generator, under some conditions). Average thermodynamic efficiency is even higher than for diesels.
Also, the Prius HSD does not have the power loss of a clutch or torque converter.
Also, the vehicle is aptly named a hybrid, which means even at highway speeds, the battery is contributing power, it's not like it alternates between the two power sources, it's using both at the same time.
With all those factors in play, is it really that surprising that (using myself and a friend as a basis) the Escape Hybrid gets 263% the fuel economy over a convential Escape?
Also... The Toyota Prius/Ford Escape hybrids use an Atkinson/Miller type engine that decreases pumping losses at low RPM as well as a dramatic reduction of emissions. This has the side effect of diminishing the torque, but pretty much all the torque comes from the large electric drive motor, especially when launching the vehicle. Another factor that makes hybrids appear more efficient is the low rolling resistance tires that they include, although this could easily be applied to non-hybrids to improve their mileage, it's usually not.
For the best discussion I have ever seen regarding this question, head over to Graham Davies' website that discusses the Prius, but general ideas are true for any hybrid.
An oversimplification would be: downsize the engine to decrease pumping losses (and add a battery for high power output requirements). Then add a host of tricks that each give more MPG improvement, such as electric only at low speeds, aerodynamic shape, low resistance tyres, 'by wire' steering, regenerative braking, partial electric climate control, and automated engine loading.
The sum total would be, in the case of the G2 Prius, engine thermodynamic efficiency rarely outside of the 33 - 38% range, and the lowest power loss through the different systems of any car on the road.
The devil, as is often noted, is in the details
R2-E2, 2G Prius.
Highway/City/Husband/Wife MPG: 56.5, as of 12/2005, 26K miles
Jac Nasser, Ford President: "We are planning to launch a hybrid version of
this car [P2000] within this year [1998]. We will also make FCEV available in
2004."
One thing I have wondered about though, is why no major car companies have attempted to market cars that maximize fuel economy without adopting a hybrid drivetrain. While hybrids have a great deal of future potential, I believe that a "middle ground" car with the secondary mileage features would be great for those who are interestd in an efficient car that could be expected to pay for itself in gas savings, without the problem of the "hybrid premium" Conventional means of fuel-efficiency can benefit hybrids and nonhybrids alike.
What I would be interested in seeing would be what kind of mileage a car would get by taking a streamlined car, and equipping it with a "shift by wire" manual (ie, still has clutch and gears, for efficiency's sake as opposed to an automatic), an appropriately sized lean-burn engine with multiple camshaft timing modes, idle-stop, low rolling resistance tires, instant mileage display, etc.
Adding features like VCM and/or lean burn to higher-performance cars also seems like a no-brainer to me.
I would be interested to see exactly how much of a difference the expensive battery-pack and motor system truly makes.
Also... The Toyota/Lexus/Ford hybrids all use an Atkinson/Miller type engine that decreases pumping losses at low RPM as well as a dramatic reduction of emissions.
Are you sure? I seem to recall reading somewhere that the Lexus (and therefore Toyota Highlander) wasn't Atkinson-ized.
Why do hybrids get better mileage?
