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Let us suppose that, through a combination of forces including mileage or emissions requirements, fuel prices, and technology maturation, it becomes de rigour for vehicle manufacturers to focus their energies on electric vehicles with extended range capacity (EV-ER): afterall, such vehicles sooth range-anxiety by carrying some energy-dense fuel instead of a huge number of heavy and expensive batteries. Manufacturers' seem to know the EV side of the equation -- sure, battery chemistry has ever-changing flavors, yet the configuration of the balance of the system (motor, controller, charger, management system, electric peripherals, etc.) are pretty well decided. However, we don't have a known ideal extended range device to provide motive power and/or electricity. What are manufacturers' options, and which is best?
Clearly, there are a variety of options: 1. conventional ICE; 2. diesel; 3. HCCI; 4. rotary; 5. turbine; 6. sterling; 7. two cycle; 8. split intake and power stroke designs; 9. ethanol fuel cell; 10. hydrogen fuel cell. And, there are just as many factors to consider in determining the pros and cons of each: 1. does it define whether the vehicle will have parallel or series hybrid architecture (i.e., is the range extender capable of moving the vehicle directly (parallel design) or can it only be used to produce electricity for the batteries/motor (series design)); 2. manufacturing cost; 3. fuel economy; 4. fuel flexibility; 5. size; 6. weight; 7. noise, vibration, and harshness (NVH); 8. longevity/maintenance; 9. usability in the full spectrum of real world conditions; 10. implication for battery pack size. Because of all of these elements to consider, let's do a slice-and-dice spreadsheet.
Clearly, the first critical decision is parallel versus series architecture. If a vehicle is to be used for driving long highway miles, then direct mechanical drive would seem an advantage: otherwise, you have to accept the Rube Goldberg-esque process of taking mechanical motion, converting it into electricity, shunting that electricity through a controller, storing it in the battery, pulling it back out of the battery, pushing it back through the controller, and running it through the motor to once again get mechanical motion -- and accept the loss of energy in each step. But, engineers might point out that this allows the engine to run at optimally designed speed and load and therefore run efficiently enough to make up for the electrical losses, and besides this way there is no need for a mechanical transmission. It also vitiates the need to engineer for all those annoying on-throttle / off-throttle / part-throttle situation, and therefore NVH, fuel management, driveline lash, and a host of related issues are sidestepped. However, series design also requires a commitment to a large battery pack, with its attendant cost and weight issues.
But wait, there's more to consider: interestingly, through-the-road parallel does away with the need for 2 motors: a point for that type of parallel, and also provides for 4-wheeldrive capability to boot. A smaller drive motor can be used in parallel because the engine can help -- which would be a point for parallel, but if the engine is an ICE and it only helps now and then, it can't heat up its catalytic converters; a point for series? Additional considerations include anticipation of improved batteries, which would slot nicely into series design; the lower top speed or need for a more expensive motor in series designs due to the motor's single-speed transmission; and the modularity of a series design that would enable simply more batteries to be used instead of an engine and broaden the variety of potential range extenders. This last point may be the convincer for manufacturers: one platform will enable both pure battery-electric vehicles and a choice of range extenders based upon fuel availability choices or technology advances.
Once a series design is opted for, and assuming there is reasonable exploration of range extenders, what do we think we'll find? The spreadsheet demonstrates factors to consider: for instance, if the cost would come down, then direct ethanol fuel cells could be great -- but they obviously require pure ethanol, which is simply not available and won't be without a real commitment. Sterling engines could be great, but they're difficult to manage due to their slow light-up: and if they're in series with large battery packs that will be primarily charged from the grid, does the high fuel efficiency matter that much? Turbines have similarly slow light-up and their lower efficiency may not matter if grid-charging the pack, but they have the advantage of being able to burn any fuel. Trade-offs abound.
Purpose-designed series engines, such as Lotus' "Omnivore" two-cycle, seem a good way to go: relatively conventional manufacture, high efficiency, not too weird for existing technicians (we stopped calling them "garage mechanics" when they started charging more than any blue-collar job you can name), and potentially small and light. But, it's not really an omnivore, as it does not profess to be able to burn diesel. Yet with the right manufacture, perhaps it could be designed to live up to its name, and then it could suffice for all possible users.
In the final analysis, the manufacturers are going to have make decisions that will effect the planet's travelers, and while everyone wants to look smarter than the other guy and stand somewhat apart from the other guy, no one wants to drive down a dead-end road. Therefore, given the comfort level existing engineers have with internal combustion, I think it likely that the future will be series hybrid architecture to satisfy mass manufacturing, marketing, and modularity concerns, with one or another version of a small, light, efficient, fuel-flexible, and semi-conventional range extender. Vive la difference, as long as it's not too different.
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I personally think technological advances in IC engines will make most of this article obsolete. Most apparent on the horizon is the "HCCI Linear Generator" (Homogeneous Charge Compression Ignition Linear Generator), which can theoretically double the efficiency of the Otto cycle and produce electricity from a broad variety of fuels with only a single basic reciprocating moving part per cylinder pair. It is expected to outperform the efficiency of fuel cells with hydrogen, and be amenable to any fuel from natural or artificial gasses, to ethanol to heavy fuel oil. Still needs some development work esp. on control systems, but with companies like GM and the big Japanese companies working on it, it should be part of every hybrid vehicle before 2020.
Jason Jungreis 6.15.10
Len,
Please note that I included HCCC engines in my article. I have researched the subject and found nothing to suggest that HCCC engines can "double" efficiency (although of course that is a vague expression, as it depends which Otto Cycle engine you are discussing: are we talking about a 1920 Model T engine (perhaps 15% efficient), or a modern direct injection turbocharged engine running at a single optimal speed (perhaps 35% efficient)?). Further, I am unaware of anything suggesting that HCCC engines can burn "heav fuel oil" and indeed I find that unlikely given the narrow operational parameters of HCCC engines. Still, HCCC engines are in the mix, and may prove preferable primarily for the reason that they require little manufacturer retooling.
Thanks.
Jason
Don Hirschberg 6.15.10
After going to a couple web sites I realize I had heard the HCCI story long ago. The resurgence of interest is because electronics might well control such an engine which has never been adequately controlled in the past.
Caution: Neither the Wikipedia nor the About.com article was technically correct.
The HCCI engine is essentially an attempt to improve efficiency and reduce emissions. If the r can be raised significantly the e of the HCCI engine could be slightly higher. But there are still unsolved problems.
The IDEAL, i.e. max, efficiency of an Otto cycle engine depends on one and only one variable, the compression ratio since the ideal engine has no friction and no pumping losses. (lots of big valves, huge manifolds, tiny bearings with frictionless lubrication, etc. and perfectly mixed air and fuel, stoichiometric combustion - in other words the mechanically perfect engine.)
e = 1 - 1/r^k-1 where r is the compression ratio and k is the specific heat ratio of air. k= 1.4 (cold air std, or k= 1.3 hot air std.. Remember the e for an ideal diesel engine at the same r is always :ESS than for a Otto cycle engine. The greater mpg experienced by diesels is partly because a gallon of diesel weighs more than a gallon of gasoline. About 7+#/gal x 20,000 BTU vs about 6#/gal x 20,000 BTU/#., and partially as the result of a MUCH higher r.
Any mechanical improvement such as better fuel/air mixing has no effect on the ideal e. Like better bearings it might nudge the real engine a bit toward the ideal engine.
It is humbling to realize that the max.efficiency of engines was calculated before the first ones were built.
Malcolm Rawlingson 6.15.10
Well written and interesting article Jason. I agree with you that all-electric cars will be limited by the battery for a long time to come. There is a lively discussion ongoing on a similar article.It would seem to me that a small lightweight engine charging a battery via a generator (the series model) is more likely to be successful. The important point to note is that it is not just the efficiency of the engine that needs consideration it is the overall weight of the vehicle and the amount of fuel needed to move the extra weight. The simpler and lighter the drive train the less metal there is to haul around.
This is why for small cars diesels are really not that much better in terms of miles per gallon.Diesel engines are heavier than their gasoline counterparts since they need much heavier construction to deal with higher compression ratios. So what you gain in efficiency you lose in power to weight ratio.
Simple fact is lighter cars do more miles to the gallon. So you need a lightweight battery operated car with a small lightweight engine to charge the battery. The less mechanics there are the less there is to go wrong...but as noted on other posts I fear that the auto industry will make them as complicated as possible so there is in fact more to go wrong...that way the profits from fixing it all will remain the same as it is now.
Malcolm
Don Hirschberg 6.15.10
In my post above "r" means engine compression ratio. "e" means thermal efficiency (and " :ESS" means LESS.)
Jason, how come you know about energy?. Young engineers I meet are most likely to think first of electricity and "power supply" in terms of milliamps. If they had ever heard of generating plants, motors and engines they seem to have forgotten about them. And you are a lawyer! I am impressed.
Maybe I am the only one, but these old eyes have trouble reading the chart in your article.
David Bruderly 6.16.10
Engine researchers at Daimler & BMW & academia believe they can use the lean burn properties of hydrogen to push efficiency of ICEs to @ 50+% with near-zero emissions. Similar results for ICEs burning natural gas - hydrogen blends -- any fuel engine system must have very low lifecycle carbon and critieria emissions --- otherwise what's the point?
The only cost-feasible, low carbon motor fuel available in scalable quantities anytime in the near future is NATURAL GAS. Off-the-shelf proven technology; the only barrier is unwillingness of vehicle manufacturers to mass produce vehicles designed from the wheels up to use this clean, safe, abundant and affordable gas as a motor fuel. And don't tell me that methane has a storage problem; the problem is between the ears of the executives who refuse to allow engineers to design and mass produce these low carbon, clean vehicles so they will be affordable.
Don Hirschberg 6.16.10
David, but unfortunately there is no hydrogen on this planet. None. All our hydrogen is man-made. Manufactured hydrogen is now and will be forever doomed to yield less energy when used than was consumed in its manufacture.
The thermal efficiency of an ICE engine is not a function of the fuel. Only the compression ratio for the Otto cycle and the compression ratio as decreased by the cut-off ratio in the case of the Diesel Cycle engine.
Len Gould 6.16.10
Well, there are a few as-yet-unimplemented tricks to improving the efficiency of the ICE. 1) reducing the fuel rate for a given expansion rate. 2) increasing the rate-of-burn post ignition. Both Otto and Diesel cycles suffer from 1) cost constraints on making the expansion rate very high, and 2) a too-slow rate of charge ignition at TDC, and an inability to mechanically handle the stresses of an ideal charge ignition at an ideal compression ration, >33:1 An HCCI linear-generator deals well with all issues as well as providing <10% of typical NOX, at present needing only the refinement of the control electronics to handle an occasional ignition failure at max lean burn rate. It does suffer from a higher rate of hydrocarbon emissions at certain loading levels due to poorer burn rates near cold combustion chamber walls, and that defect may actually kill it. If so, too bad.
Jim Beyer 6.16.10
For a PHEV, the efficiency of the IC engine is a secondary and perhaps even tertiary concern. Main issues for the engine instead are cost, weight, and reliability. Why? Do the math.
If a PHEV with an all-electric range of 20 miles can remain all-electric 80% of the days it is driving, then the fueled portion of the drives will end up being a very small percent of the overall mileage, perhaps 10 percent or less. Given that, applying resources to the efficiency of IC engine produces little return. The main purpose of the IC engine in a PHEV is to provide adequate range and continued operation (no recharging worries) for the times that the vehicle IS driven long distances.
For vehicles that are routinely driven long distances, they either need a battery sized to match this (expensive, especially if it's not used often) or should just be a convention IC or HEV vehicle. Then better engine efficiences would have merit.
Graham Cowan 6.17.10
Maybe I am the only one, but these old eyes have trouble reading the chart in your article.
Did you click through to the larger versions? Still fuzzy, but not quite so much.
What concrete example does Jungreis have of a "Sterling" engine with conversion efficiency exceeding 25 percent?
All this business about increasing the efficiency of ICE engines is old and fatuous news. Let me digress. In the 1920's some fancy carburetors had a selector switch labeled RICH , NORMAL, LEAN. Without electronics this was quite an accomplishment. RICH meant about 13 pounds of air per pound of fuel and LEAN meant 16 pounds of air per pound of fuel. Chemical engineers had long ago calculated that it takes 15 pounds of air to combust a pound of gasoline,
Like the multiplication tables thermodynamics has not changed. That is, what we thought true about energy a hundred years ago is true today. Precisely.
Those seeking nirvana by efficiency improvements are advised to look elsewhere.
Jim Beyer 6.18.10
Getting electricity from a fuel seems to top out around 35%. HCCI might get you to 50%. We'll see. Fuel cells are not Carnot limited, but they do have to respect Gibbs Free Energy limitations. Practically, fuel cells are problematic because their power output is proportional to a membrane size (2-D) whereas IC engines' power output is proportional to piston displacement (3-D). Easier to increase volumes than to provide more membrane.
On the fuel PRODUCTION side of the equation, electrolysis tops out around 70% efficient. It is also membrane (2-D surface) limited, so increasing efficiency (lower current per unit area) means bigger cathodes/anodes and thus higher cost.
Once you got the hydrogen, creating hydrocarbons is fairly straightforward. You are about 80% efficient in creating an H-C bond for every H-H bond input. This results in a denser fuel which is easier (sometimes much easier) to transport and store. So overall fuel production efficiency from electricity is 50-60% on a good day.
The 'roundtrip' efficiency (electricity-to-fuel-to-electricity) is therefore 10%-30%, depending on the overall system. 20% is probably a more realistic goal.
Anumakonda Jagadeesh 10.26.10
Excellent article by Jason Jungreis, What kinds of electric car motors are the most popular? There are a lot of electric motors out there, but just a few make up the majority of the motors being used in electric cars. When people choose a motor for their car, they're balancing factors like cost, availability, and the "do it yourself factor" - the confidence that they can get all these expensive gizmos installed and not just wind up with a project scattered all over the garage...a well-known cause of marital discord! The Good: All the torque is available from a standstill. You've no doubt heard about this famous characteristic of electric cars. It's really a characteristic of these series wound DC electric car motors, though, rather than electric cars in general. These motors happen to be popular for electric drills, too, and that's why. The Bad: Not your best hill-climbing device. The Electrical Engineering training series says, quote: "Series motors cannot be used where a relatively constant speed is required under conditions of varying load." Unfortunately, that pretty much sums up what an electric car would be doing in hilly terrain, like my neighborhood. The DC series motor in your NEV might not climb those hills too briskly. Oh, you noticed this already? So did Zenn and Miles, actually...and this year(2008), their new cars have AC drive systems installed. The Ugly: These always have to be run under load. Meaning, when you're installing your shiny new Netgain motor in that Chevy S-10 you've so carefully de-ICEd, you don't wanna hook the motor up to 120 volts and just whizzz it out attached to nothing (you know, to hear what it sounds like running); it'll take your head off. Apparently it turns with enough force to even damage itself if you run it without a load! The Advanced DC Series wound motor is by far the most popular of the electric car motors, according to the EV Photo Album, followed by GE and Netgain.(Source: Electric-Cars-Are-For-Girls.com.)
DC Motors There are four main types of DC motor, namely permanent magnet, series, shunt and seperately excited. The latter three all use field coils in the stator (the part which doesn't move) to generate a magnetic field for the rotor to spin in, and their name simply refers to the way the field coils are wired with respect to the rotor coils. All four types use a commutator to control which rotor coils are energised at any given time in order to maintain rotation, and it is enough just to apply a DC voltage across their terminals to get the motor to spin, so they are relatively easy to control. Currently series DC are the most economical and commonly used type of motor in electric vehicles. Being a tried-and-tested technology, they are actually quite good – with efficiencies up to 90% and only needing servicing every 100,000kms or so. However using a commutator is restrictive and a source of inefficiency. Also, with series DC motors regenerative braking is very difficult to do (in fact, you basically have to operate the motor as a sepex DC motor). Regen can increase your range by 10-20%, so is quite valuable. (Source: ZEVA,Zero Emission Vehicles Australia).
