230 mpg in the Volt? Chevy fudging the numbers?

[chuckm]

Ok, you try getting 50+ with your engine while you're charging a *whatever* amp hour battery pack at full load...

That was kinda my point. In conventional engines, we have traded efficiency for flexibility. I couldn't get that kind of efficiency in my car. But if an engineer is told, "Hey, design me a super-efficient engine. It needs to output XhP at Yrpms. No load or speed variability, just on or off," you'd end up with a very efficient design. Probably a miller or atkinson cycle, though a brayton engine would not be out of the question (just much more design intensive). In fact, for a fixed speed and fixed load, brayton would be ideal (though not Carnot ).

[cfg83]

Hello -

GM claims to be using an "early version" of the EPA Standard :

Chevrolet Volt should get 230 mpg in city, GM says -- latimes.com

The automaker's fuel economy estimate hasn't been confirmed by the EPA, which is developing a new methodology for calculating fuel economy ratings for cars that can travel significant distances powered only by electricity. GM said it used the EPA's preliminary guidelines in developing its mileage estimates for the Volt.
The EPA publishes mileage estimates for vehicles sold in the U.S. based on city and highway driving, as well as a combined city-highway mileage estimate.
GM said it had calculated a highway mileage estimate for the Volt but didn't release the figure. The automaker said it was confident the car's combined city-highway fuel economy "will be in the triple digits."

When the EPA finalizes it's standard, I'll bet the MPG will be much lower, but still in the triple digits.

CarloSW2

[KJSatz]

Well it sounds like it gets roughly 50mpg when it's running the generator to replenish the batteries as they are used up...really what they need to do is see what it gets in the city and highway (current EPA tests) but in mi/watthour (or whatever). Then you get two pairs of results.

All-electric: range in the city (miles), range in the highway (miles)
After all-electric: city (mpg), highway (mpg)

That is the only way to get a complete picture. It won't be as cool sounding as 230mpg, but that's really misleading.

[jamesqf]

Originally Posted by Christ

Ok, you try getting 50+ with your engine while you're charging a *whatever* amp hour battery pack at full load..

OK, if you'll tell me why a competent engineer would design the system so that it charges while at full driving load. I don't know exactly how GM has implemented their system, but if I were doing it, it would blend gas & battery use, and do most of the charging on downhills or when braking.

[dcb]

The engine should not charge the battery, at least not very much, that is what the house plug is for. and would be a waste of gas to do so.

The engine output is actually less than the motor at peak, so top speed and acceleration probably drop off, plus no means to cruise at bsfc without using the engine to drive a generator to drive a motor when the battery is shot.

I'll take a 75mpg corolla for long trips for 1/10 the money, thank you

[NeilBlanchard]

Hi,

The Volt is what is called a serial hybrid (though the folks at GM want to call it an "extended range EV"). This means the electric motor pushes the car by it self, and the ICE is used to charge the battery -- this is similar to how electric/diesel trains work.

So the ICE should be run at one constant -- and it's most efficient RPM, to charge the battery. They used a 1.4L 4-cylinder engine, which is way too big for what it needs to do. It should have been a 500-750cc (tops!) and even a smaller ICE could have worked.

The too-large engine has a major weight penalty.

[cfg83]

Hello -

More of what Neil is saying :

Hybrid Center :: How Hybrid Cars work :: under the hood 2

Series Drivetrain

This is the simplest hybrid configuration. In a series hybrid, the electric motor is is the only means of providing power to get your wheels turning. The motor receives electric power from either the battery pack or from a generator run by a gasoline engine. A computer determines how much of the power comes from the battery or the engine/generator set. Both the engine/generator and regenerative braking recharge the battery pack. The engine is typically smaller in a series drivetrain because it only has to meet average driving power demands; the battery pack is generally more powerful than the one in parallel hybrids (see below) in order to provide remaining peak driving power needs. This larger battery and motor, along with the generator, add to the cost, making series hybrids more expensive than parallel hybrids.

While the engine in a conventional vehicle is forced to operate inefficiently in order to satisfy varying power demands of stop-and-go driving, series hybrids perform at their best in such conditions. This is because the gasoline engine in a series hybrid is not coupled to the wheels. This means the engine is no longer subject to the widely varying power demands experienced in stop-and-go driving and can instead operate in a narrow power range at near optimum efficiency. This also eliminates the need for a complicated multi-speed transmission and clutch. Because series drivetrains perform best in stop-and-go driving they are primarly being considered for buses and other urban work vehicles.

CarloSW2

[Christ]

Originally Posted by jamesqf

OK, if you'll tell me why a competent engineer would design the system so that it charges while at full driving load. I don't know exactly how GM has implemented their system, but if I were doing it, it would blend gas & battery use, and do most of the charging on downhills or when braking.

When did I say it was at full driving load?

The engine doesn't drive the car... what part of that isn't understandable? The engine charges the batteries when they're completely drained (Pre-damage drained), because it produces more power than is necessary to drive the car under most, if not all, circumstances. In this way, it is capable of operating at full load all the time, thus being at it's most efficient power setting for most of it's operation.

During the charging/driving cycle, the engine is STILL capable of delivering 50 miles per gallon of fuel used, but the mileage used when the engine is off is technically "free", since you already paid for it while charging the batteries while driving.

[NeilBlanchard]

Hello,

Here's another possibility; convert the electricity to a gasoline equivalent. From Wikipedia:

Electricity[4] 33.40 Kilowatt hours * 3,413 BTU/kWh[9][10]

*calculated based on 114,000 BTU/gallon base gasoline

So this yields ~83.5MPGe for the first 40 miles on the Volt, and then ~50mpg after that. Instead of 230mpg on the same 50 mile trip, the Volt gets 73.6mpg -- which is completely plausible and believable... why didn't GM choose this method? Sheesh.

[dcb]

They could cover 95% of users needs by replacing the gas and generator crap with more batteries, and get to market years sooner.

Put a trailer hitch on it and sell a range extending trailer when you get the gas stuff sorted (that can hold the rest of your vacation stuff too), but please get in the EV game as soon as you possibly can GM (and Ford and Chrysler, and Harley, and anyone else with some gumption). Quit dinking around with gas, it is killing you, should not be holding up the release of an electric vehicle.