Series Hybrid powerplant options?

[dalcazar]

I recently read this article

http://www.wired.com/autopia/2010/10/the-chevrolet-volt-isnt-a-true-ev/

What struck me was the fact that the volt is a series hybrid, something I didn't know about the car until I saw this article. What grabs my attention isn't that the car isn't a true EV, that discussion is pointless and just a bunch of ammo for people with nothing to do.

My question became, of all the engines that GM could have put in this thing to power the series hybrid system, it's gone with what I would consider the worst of all options. Now, I know that you can't introduce a diesel powered car in the US and that using a turbine would be a recipe for disaster.

My initial impression of the Volt was that I might want one, but after having a look at how it works and what it's real MPG figures are, coupled with the massive price tag, I don't. At all. This thing seems like nothing but a PR gimmick by GM that gets worse gas mileage than most diesels and even many conventional gas powered cars. The electric range is too short to be useful to more than an urban commuter and the amount of weight you're hauling around when you run out of charge, coupled with the inefficiency of an IC to Generator to electric drive system, seems to affect the car to the point that it's inferior to all normal hybrid drive cars, and even some standard cars.

Now my question: I haven't kept up on the times of small scale gas turbines, but would the use of either a gas turbine or a diesel engine make this car a better platform? Wouldn't the use of either of these options enable you to use an engine that got better "miles per gallon" and at the same time allow enough excess power to charge the batteries whilst driving? The weight savings or extra efficiency alone seem to make it a much stronger contender.

 

[BrianPetersen]

Gas turbines have TERRIBLE off-design (part-load) thermal efficiency and they don't scale down to automotive-scale power levels all that well due to Reynolds number effects (fluid dynamics). In order for turbine engines to be efficient, you need multiple stages of regeneration and reheat, and the pressure ratio across each compressor / expander stage needs to stay in the ballpark. This sort of thing is practical on a utility-power-generation scale where you are talking about megawatts of power. They don't like running at off-design speeds (your utility power is either 50 Hz or 60 Hz depending on where you are in the world, but it is ALWAYS that frequency), they don't like changing load rapidly, and they don't like starting and stopping. Works OK in aircraft where they can also run at a high load level for hours at a time.

If you don't think this is an issue ... the average power demand for driving the car at 70 mph is an order of magnitude different from that required for driving 20 mph stop and go. So, what do you do with this hypothetical turbine that doesn't like varying its speed and doesn't like starting and stopping very often?

I know of no automotive-scale turbine that will match the efficiency of a good piston spark-ignition engine in real world conditions, nevermind even coming remotely close to touching the efficiency of a diesel engine.

It's safe to say that a turbine in a practical real-world automotive application is "out".

A diesel, on the other hand, would make a good match in this application. But ... GM doesn't have a production diesel engine that would meet North American Tier 2 bin 5. (Yeah, other manufacturers do ... but GM doesn't. Contrary to your statement, you can buy a diesel powered VW or BMW today, in the USA, even in California, so your statement that you "can't introduce a diesel-powered car" is not correct.) The cost of the additional emissions hardware needs to be weighed against the cost of fuel. Remember that the Volt is "supposed" to be driven as an electric car, and the gasoline engine is not "supposed" to be used at all except for the occasional longer trip. How true that will be in the real world, of course remains to be seen.

The four-cylinder 1.4 gas engine that they picked is not a bad choice considering all of the factors - including minimizing overall powertrain cost. That is an existing production engine in GM's worldwide lineup - so they didn't have to engineer a completely new engine for this application, which presumably would not have been feasible in terms of cost or schedule.

 

[MikeHalloran]

Given the presence of two sets of conversion inefficiencies, it should come as no surprise that a series hybrid doesn't get spectactular fuel mileage.

My guess is that some people at GM knew just how unspectacular the numbers would be for a long time, hence all the 'equivalent mpg' mumbo-jumbo, but the project had so much visibility that changing course would have embarrassed too many bigwigs.

WRT gas turbines, I had occasion to compare a modern 40kW gas turbine to a not quite modern 40kW Diesel, in a generator application. The fuel consumption of the gas turbine was ~15 pct higher than the Diesel at all power levels, and it was only that good because of the presence of a recuperator, which added to the package volume.

One other problem with a gas turbine is that you need a really big exhaust pipe, on the order of 6" or 8" diameter at 40kW, which presents packaging difficulties in an automobile.



Mike Halloran
Pembroke Pines, FL, USA

 

[dalcazar]

The beauty of having a series hybrid is that engine load doesn't necesarily have to have anything to do with actual vehicle speed. Yes you have vastly different loads at different speeds, but when idling and/or decelerating, the power production from the engine can be re-routed to charging instead of powering the generator, so if you're sitting still the turbine can run a full capacity and just charge the batteries. Go off the line and the power transfers to the wheels instead of the battery. I don't know much about electronics and just how hard a switching conveter of this design would be to design... but that rather negates the problem of loads.

As to the issue of introducing diesels to america, I was going along the lines of the general american public being about as enthusiastic about buying diesels as they are with buying milk in tetra-paks. It's an altogether better technology, they just don't like doing it because of idiosyncracy.

So turbines are not as fuel efficient as diesels, I was rather expecting that statement to be true but wasn't aware of current technologies

 

[GregLocock]

Actually it is a series/parallel architecture. And it is a true EV until the battery runs down.

I'm glad they are building enough to investigate whether it is a viable alternative. If I were to have one then it would halve my fuel bill each week, tho the electricity bill would soar. However the cost of fuel is a fairly small part of the TCO of a new vehicle.





Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

Series-hybrid fanatics (most of whom, I would classify as "greenies" - and the derogatory term is purely intentional) always tout the engine load being independent of vehicle load as an advantage.

The problem is, you have to consider the vehicle's complete system as a whole, not just the engine.

If you use a gear-to-gear transmission, the engine has to run at least somewhat "off design" (away from its best-efficiency speed and load conditions) a lot of the time, but from mechanical power "in" to the transmission to mechanical power "out" to the wheels can be at least 90% and possibly as much as 95% efficient as long as you don't throw it away with inefficient torque converters and hydraulic pumps (I prefer to row my own gears - manual transmissions don't have any of that stuff).

In a series hybrid in which the engine cannot transmit power to the wheels mechanically and in which the engine load is fixed, there is almost no choice but for the powertrain to be either charging or discharging the batteries ALL the time, because the vehicle load would only rarely match generator output exactly. So then, rather than the (at least) 90% efficient gear-to-gear transmission, you have:
3 phase AC alternator - approx 92%
Rectifier - 99%
Battery charge and discharge - 85% (note: the charging voltage of a normal lead-acid battery is around 14 volts and the discharging voltage is a little over 12 volts, and that doesn't include current leakage and doesn't include extremely high charging or discharging demand, in which case the discrepancy is higher than this)
Inverter - 95%
AC motor - 90%
Gear reduction (yes, you still have that!) 95% (I will grant this higher number on the grounds of not needing multiple speeds in the gearbox)
Total (multiply all of these together) ... 63%!!

So, sure, the engine is running at its best efficiency point, but then you throw away that valuable mechanical work by converting it between several different forms in order to get mechanical work out at a different rate.

The upshot is that if you use a mechanical gear-to-gear transmission and let the engine speed and load vary, even to the point of sacrificing a third of the energy, you will be ahead of the game compared to a series electric powertrain!

So ... guess why the Volt varies the speed of its engine (to at least somewhat match demand - it cuts down on the battery charge/discharge losses) and allows the engine to be at least partially mechanically coupled at highway speed (to cut down on all the energy conversion losses).

And ... guess why a Prius is designed to transmit engine power mostly mechanically at highway speed. (It does so to a MUCH greater extent than a Volt does.)

Series hybrids just ain't the hot ticket in this type of application. If there's an engine involved, and you want good efficiency out of the complete system, you had better be able to couple it to the wheels to bypass all the electrical energy conversions and their associated losses.

 

[DanEE]

The most recent, sometimes acidic discussion of whether the Volt is what GM said it is... is due to the poor judgement by GM of releasing 3 years of hype before the design is done.. Always a risky, bad, potentially backfiring decision when dealing with b(leading) edge technology attempts..

My main career was with a major technology company and was directly involved in the decisions of readiness for product announcements, release to production, and start of customer shipment... If there's any tinge of delivering less than what has been promised, not only is it reasonable to expect bad PR in a competitive market, but you may well find anti-competitive lawsuits on your door step due to pre-announcing a "paper tiger"..

While GM will finally deliver, whether production volumes will ever ramp up enough to bring the $41,000 cost down and whether their implementation of a car carrying around 435 lbs of batteries that only gets about 30mpg after the 25 - 50 mile range is depleted, well the market will decide. Its 4 passenger seating only (due to battery intrusion into passenger space) will probably make the car to be classified as a compact (versus 5 passenger seating typically in the midsize classification cars). Weighing in at 3980 lbs is heavy for a small car.

btw independent tests of the Prius plug in hybrid (in field testing for the past year or so) to be in the marketplace in 2011, with only a 12 - 13 mile electric only range, gives the car enough energy buffer storage for the current HSD engine/parallel/series hybrid technology to deliver mpg in the 80mpg range..

Ford's new Fiesta with direct inject engine technology, 6 speed sequential shift dual clutch transmission will be in the 40 mpg range..

An of course diesels are getting better and better..

The competition doesn't stand still..

 

[dalcazar]

I'm still waiting for the diesel-electric parallel hybrid. I'm surprised they haven't come out yet. So long as series hybrids can have a realistic range under electrical power (100 or so miles) I see no fault with the series hybrid configuration.

I like the idea of the series hybrid, a while back I was toying with the idea of converting a corvette to series hybrid running a microturbine, but that was a pipe dream that only lay within the realm of cool and not reality.

What will kill the Volt is the fact that it will spend 95% of it's life running on gasoline and not power from the plug. I see this car as a premature birth that will probably die and cost GM more of it's already bad reputation.

 

[duck0601]

Unless something has changed in the last couple of years, the Volt was a series hybrid, with the engine coupled to a generator which charged the batteries. There is no mechanical linkage between the wheels and the engine. GM's series/parallel system was in their Yukon/Tahoe/Escalade vehicles and also one of their pickups.

 

[BrianPetersen]

Something HAS changed in the last couple of weeks (actually, it changed sometime in the last couple of years, but it wasn't public until a week or so ago). The Volt DOES have capability of the engine mechanically driving the wheels - only if it is already in range-extending mode and only at highway speed.

GM obviously recognized that pure series operation is too inefficient.

 

[GregLocock]

It's mode 3 in figure 3a of their patent

http://www.freepatentsonline.com/20090082171.pdf

. Parallel mode, with both the IC and the motrs linked to the wheels, can be used at speeds as low as 30mph, and is always used above 70mph, once the battery is depleted.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[duck0601]

Ah, my mistake then. It's what happens when you drop out of the loop for 18 months. I didn't see the report on the change to the high speed charge depleted mode. The buzz on the news here was no mechanical connection ever, and fit with the information I had April '09.

Practicality, I would say, depends on the drive cycle, distance and gas prices.