The Efficiency Of Steam And Other Cars


UPDATE -- Saturday, February 23rd, 2002

Several bits of important new information have come in since the following material was written. I have obtained road efficiency charts for typical gas cars from industry sources, showing 7% city and 10% highway overall thermal efficiencies (fuel tank to wheels) at cruise. This is lower than I had expected. My higher figures below, however, assume that the gas engine runs at full load on the highway, and no actual production engines do this because the much smaller engines required (30-40 hp) are unacceptable to modern drivers. So perhaps I should have expected the lower figures.

Oddly, I have also discovered test results showing that the cruise efficiency of steam cars with system parameters like the ones I am planning, is also lower than I had thought -- but it is _identical_ to that of production gas cars (7% city-speed cruise, 10% highway cruise). The steamer will still get better fuel mileage in the city, though, as it has no "idling" at stops, no "riching" of fuel/air mixture during acceleration, and no fuel consumption during deceleration or "coasting". Enriching fuel/air mix during acceleration cuts the gas car's efficiency, while the steam car engine's higher steam pressure and temperature during acceleration _increase_ its efficiency.

So, in identical vehicles in identical stop-and-go driving, the steam car powerplant uses less fuel than the gas car powerplant. At cruise, the fuel use is the same. Importantly, as noted below, most driving is stop-and-go driving, and the predominance of stop-and-go driving will increase in the future due to increasing urbanization and road congestion.

I have also learned that production photovoltaic cells are now more efficient (14%) than I had thought (I was using older figures), and that electrolysis is now about 65% efficient rather than 25% (older figures again). However, these recent improvements still fall far short of making solar-generated hydrogen fuel, or fuel cells running on it, economically competitive with other fuels or motors. The equipment is still too expensive, and will remain that way despite likely improvements.

These updated figures should be kept in mind while reading the older material below. My conclusions about the relative advantages/disadvantages and likely futures of the various automotive drive systems remain unchanged.


UPDATE -- Tuesday, January 30th, 2001

The February 2001 issue of Popular Science magazine, page 34, notes that methanol reformer fuel cell cars are currently about 22% efficient, and that the 40% efficiency I quote for them in the below article is currently only a theoretical (maximum possible) goal for improvement. These figures were provided by the fuel cell developers themselves.

PS also says (again quoting the fuel cell developers) that reformer cells' current efficiency of 22% is "about the same as a diesel engine mated to a conventional transmission", which confirms my diesel car efficiency calculations below. I calculated that the efficiency of the best (VW) conventional diesel cars is about 21% in the EPA city driving cycle (city driving accounts for most road mileage).

The same Feb 2001 Popular Science, page 62, confirms that the ultimate stumbling blocks to hydrogen fuel cell cars are that the fuel cells cost several times as much as a conventional gas engine, and that there is no infrastructure for producing or distributing hydrogen fuel.

I would add that a solar hydrogen fuel cell car would be at best 6% efficient overall (40% sun to electricity with large-scale solar steam plant, 25% electric to hydrogen through hydrolysis, and 60% hydrogen to horsepower at wheels, neglecting some additional losses in the conversion of potential energy). With photoelectric cells, the overall sun-to-horsepower efficiency would be more like 1.5%. In contrast, the solar heat battery steam car noted elsewhere on this website (see website index) could easily be 12% efficient overall (sunshine to horsepower at wheels), meaning half the area of solar collectors needed for the same vehicle miles, and without the considerable expenses of the solar electric plant, hydrolysis cells, and hydrogen fuel processing equipment. Plus the heat battery steam car powerplant (with 200 miles or more driving range possible) would be far simpler and cheaper than the hydrogen fuel cell system.

Thus, even in a zero-emissions, no-fossil-fuel world, the steam car is a more promising technology.


And now, the original article:


The most frequently raised objection to steam automobiles is the claim that they are less efficient and use more fuel than other cars.

This objection is based on a simplistic and unfortunately widespread misinterpretation of the results obtained in laboratory efficiency tests. These tests, usually called "net thermal efficiency" tests, measure the percentage of potential energy in the fuel that is converted into horsepower measured at the crankshaft.

In laboratory tests, gasoline car engines running at maximum power are about 24% efficient. Steam car powerplants running at maximum power test out at about 15-17% efficient. So the steam car will use more fuel, right?

Not quite. The problem with this thinking is that it overlooks other, more important test results. Now run these same engines at part load. In a car, these engines will run at part load most of the time. When the gasoline car engine is throttled down to run at these real-world average-driving horsepower levels, its efficiency drops to as little as 7%, and typically to 11-13%, for a number of complex engineering reasons. When the steam car powerplant is throttled down to the same output, tests show its efficiency drops to ... 11-13%.

Thus, in city driving, the steam car gets about the same fuel mileage as a comparable gas car, or better. On the highway, the gas car gets only slightly better fuel mileage than the steamer, because even then it is still not running at the full power which is required for its 24% maximum efficiency.

Speed/horsepower charts clearly show that even large cars do not use more than about 30-40 horsepower when cruising on the freeway, and much less, often 5-10 horsepower, at lower speeds, as on city streets. Yet most gas cars have engines rated at 75-150 hp or more, a design "side effect" of the excess engine displacement needed to provide good low-speed torque for acceleration with acceptable engine life. So the gas car engine is still running at "part load", even on the freeway, and this cuts its efficiency well below its 24% maximum. This efficiency drop at part load is not something which can be "designed out"; it is an inherent feature of the 4-stroke gasoline engine. That is why major automobile manufacturers have been investigating alternative automobile powerplants in recent decades.

Despite extensive refinements, gas cars have not increased much in efficiency in the past 20 years. The 1999 Chevy Corsica gets almost exactly the same EPA-rated city/highway fuel mileage as a 1978 Ford Fairmont of the same weight, frontal area and engine displacement. This despite the fact that the Corsica has a more aerodynamic body and thus less air drag for the powerplant to overcome. The new Corsica performs better and puts out much less smog, but EPA tests measure fuel mileage in sedate, average drive cycles, not at maximum horsepower.

Some Calculations On Car Efficiency

The May, 1999 issue of Popular Science magazine gives a good starting point for calculating actual gas car efficiency. It is a well-known and often repeated fact that gasoline car engines are about 24% efficient at full power, so we can start from there and see what published information tells us.

"Hybrid" automobiles are a special new type of car, now entering the marketplace. They are designed to maximize the efficiency of the gasoline engine by avoiding inefficient part-load operation. They do this by coupling the engine to an electrical generator, which charges batteries that drive an electric motor that drives the car. They are a cross between a gasoline and electric car, hence the name "hybrid". Because their engines run at full load while charging the batteries, they can deliver close to their 24% maximum efficiency on the road, even in city driving, when the car is running at well below the rated horsepower of the engine. Under those conditions, the engine turns on and off, charging the batteries.

The May 1999 issue of Popular Science included a very instructive comparison between the 1999 Toyota Prius hybrid and the 1999 Toyota Corolla. These cars have almost exactly the same size, weight, and air drag, so in the same standardized EPA fuel mileage tests they should use about the same amount of power in horsepower/hours. So if these cars get the same fuel mileage in miles per gallon (mpg), that means their powerplants run at the same efficiency. If one of them gets twice the fuel mileage, it must be twice as efficient. And so on.
The Prius hybrid got 47 mpg in the EPA urban test.

The Corolla standard got 28 mpg in the EPA urban test.

Now, if the Prius engine runs at 24% efficiency, and the hybrid drivetrain runs at the 85% efficiency which has been reported, then the car's peak efficiency at the wheels is about 20.4%. (24% x .85 = 20.4%). If we carry these simple calculations a bit further, we see that if the Prius got 47 mpg while the Corolla got 28 mpg, then the net thermal efficiency of the Corolla in city driving is about 12% (20.4%/47mpg = 12.153%/28mpg).

The EPA highway fuel mileage figures for the hybrid Prius and conventional Corolla were exactly the same, which tells us that the conventional-powerplant gas car gets the same 20.4% "fuel tank to tire" net thermal efficiency as the gas/electric hybrid car under highway conditions.

Now, Toyota cars are just about average for the industry, so these figures can be taken as representative of gas cars in general.

To show that these figures are pretty close to the mark, consider that in the same article Popular Science (PS) reported their own test results. They drove a Prius and a Corolla, nose-to-tail, in ordinary street driving and got 57 mpg for the Prius and 32 mpg for the Corolla. This calculates out to about 11.45% efficiency for the Corolla in the PS tests (20.4%/57mpg = 11.45%/32mpg).

So we can figure conventional gas automobile efficiency at about 11.45 to 12.153% in city driving, and about 20.4% on the highway. This compares well to the 12% city driving and 15%+ highway driving efficiency of "classic", pre-1930 style steam automobiles, especially since most driving takes place under city conditions. 75% of the world's population lives in urban areas, and in today's cities, even the highways often have low-speed, stop-and-go traffic during the rush hours when most people drive. In today's world, relatively few miles are driven on open highways at cruising speeds.

What About Other Alternative Vehicles?

Due to their good city fuel mileage, gas/electric hybrid cars have been touted as the "car of the future". However, the cost and durability of their batteries is questionable. Like electric cars, hybrids are subject to certain electrochemical limitations which mean that their batteries can either be long-lived but very expensive, or else cheap but short-lived and needing frequent replacement. In fact, it has been calculated that either way, electric vehicle battery replacement, figured out to pennies per mile, costs more than the gasoline the car would use if it ran on gasoline. This fact dooms both electric and gas/electric hybrid cars.

Electric car advocates often say that mass production will bring down the cost of the batteries, but in fact these batteries have already been in mass-production for decades, and they still cost the consumer more per mile than the fuel they replace. The cost problems are due to laws of physics (electrochemistry) which are unaffected by the amount of investment or mass production devoted to the problem. Add to this the fact that cold weather can reduce the already limited (60-90 mile) range of the electric car by 30-50%, and the generally poor performance of electric cars, and it is not hard to see why few people buy them. Plus, if you have to park your car on the street, as many, perhaps most drivers do, how are you going to recharge an electric car?

As further evidence of the electric car's bleak future, Honda recently canceled their expensive electric car production project and announced that they had no future plans to work on electric cars, due to almost nonexistant consumer interest. They had put an electric car on the market, and after two years almost none of the cars sold.

Others have proposed fuel cell cars as "the car of the future". Fuel cells running on hydrogen have a net thermal efficiency of around 60%, and "reformer" type fuel cells, which can break down hydrocarbon fuels like methanol or gasoline to create hydrogen, give an efficiency of about 40%. The lower efficiency of the reformer fuel cells comes from the fact that the potential energy in the carbon content of the fuel is wasted.

Like electric cars, however, fuel cells also have certain electrochemical limitations which make them unlikely to ever be economically viable. Fuel cells are little more than very advanced and expensive electric batteries. Car-sized fuel cells cost about $30,000, compared to $3000 for the average conventional gasoline engine automotive powerplant, and it is unlikely that even mass production can cut their cost tenfold, due to the expensive raw materials and production processes required. Early results with experimental fuel cell cars are not promising. Daimler-Chrysler's hydrogen-fuel-cell Necar was road-tested by Popular Science in 1999, and reportedly had sluggish performance, limited carrying capacity, and various other problems, most related to the large size and weight of the fuel cells.

On top of that, sustainable (solar, wind, etc) hydrogen fuel production requires electrolysis of water, and only part (about 25%) of the energy used to produce the hydrogen is recoverable by burning (or fuel-celling) the hydrogen. Thus the overall energy efficiency (energy source to tire) of a hydrogen-fuel-cell car, even at 60% "fuel tank to tire" efficiency, is very low. Hydrolysis requires lots of expensive electricity, and most of the electricity is lost in the process. For this reason, hydrogen is very unlikely to be an affordable mass-produced fuel in the foreseeable future.

[See Update, 1-30-2001, above, for more on this.]

In reply to claims that electric, hybrid, and fuel cell cars are the "cars of the future", the General Motors website says it is too early to tell whether any of these vehicles can replace the conventional gasoline-engined car. This is from a company which has spent hundreds of millions of dollars in research and development funds working on these vehicles. It is a good bet that they know what they are talking about.

How About Diesel Cars?

Diesel cars are another alternative, and in fact these are in production today, and have been for several decades. The problem with diesel cars is that, for whatever reason(s), few people like them. Even during the "energy crisis" of the 1970s, with fuel station lines wrapping around the block and fuel rationing being imposed, diesel cars did not capture more than a small fraction of the car market.

According to Greenpeace, currently diesels account for only 5% of the world car fleet, and virtually all the rest are gasoline cars.

The unpopularity of diesel cars is probably due to a number of factors. They are noisier and vibrate more than gas cars, their fuel and exhaust smell bad, they are often balky in starting during cold weather, and worst of all, they tend to give sluggish performance. A major factor is probably the fact that diesel fuel is not as widely or conveniently available as gasoline. Perhaps less than 10% of fuel stations carry diesel fuel. Other problems include servicing the special fuel filters required and the generally higher price of a diesel engine of the same power as a gasoline engine. Plus there are environmental problems with diesels (soot and nitric oxide emissions), which will probably raise the cost of diesel engines even further if they can ever be solved.

The efficiency of diesels has been overstated on occasion. For instance, the latest VW diesel engines, the most efficient small diesels available, are claimed to be 45-46% efficient. A comparison of the latest, otherwise identical gas and diesel VW cars shows otherwise. Assuming from the above that gas cars are about 12% efficient in the EPA city tests and 20.4% in the EPA highway tests, we see that a diesel VW "New Beetle", at 41 mpg city and 48 highway in diesel form, and 23 mpg city and 29 mpg highway in gasoline form, gets 21.39% efficiency in the city (12%/23mpg gas = 21.39%/41mpg diesel), and 33.76% efficient on the highway (20.4%/29mpg gas =33.76%/48mpg diesel).

The new VW diesels may get 45-46% efficiency in optimum-load lab tests, but in an actual car they get more like 21% in the city and 34% on the highway. These are the best diesel figures, and it is interesting to note that some current steam car designers, like Andy Patterson and Jerry Peoples, are working on advanced steam cars which would have higher efficiency than these diesels.

A comparison of gas and diesel Golf/Jettas is less promising. The gas G/J gets 24 city and 31 highway, while the diesel G/J gets 40 city and 49 highway. This translates into diesel efficiency of 20% in city driving (12%/24mpg gas = 20%/40mpg diesel) and 31.58% in highway driving (20.5%/31mpg gas = 31.58%/49mpg diesel).

Even worse are the Mercedes-Benz diesels. The Mercedes-Benz E-Class gets 21 city and 26 highway in gasoline form, and 29 mpg city / 34 mpg highway in diesel form. This translates into 16.57% diesel efficiency in city driving (12%/21mpg gas = 16.57%/29mpg diesel), and 26.67% diesel efficiency on the highway (20.4%/26mpg gas = 26.67%/34mpg diesel).

These figures neglect the fact that diesel fuel has slightly more heat energy per gallon than gasoline. So the actual efficiency of these diesel vehicles is somewhat lower.

The most disturbing fact about the diesel experience, however, is that it shows how unimportant fuel efficiency is to consumers in the long run. Diesel vehicles can get 50% better fuel mileage than gasoline vehicles (IE, they use 2/3 the fuel), yet have always been flops in the marketplace, even during fuel shortages or where fuels are highly taxed and therefore expensive. 95% of the cars on the road are gasoline cars, despite diesel engines having been readily available for several decades. The experience with automatic transmissions gives similar evidence. Over 90% of new gasoline cars have automatic transmissions, despite several miles per gallon decrease in fuel mileage with this option. People just don't care that much about fuel mileage, even when fuel is rare or expensive. Convenience and performance are much more important to real-world drivers.

So much for the idea that losing a few mpg in today's increasingly rare highway-cruising conditions is why steam cars aren't on the market.

This also raises serious doubts about the future of hybrid and fuel-cell cars, whose only advantage, like the diesel car, is higher fuel mileage.

The worldwide market dominance of automatic-transmission gas cars seems to indicate that people prefer the quietest, smoothest, cheapest, easiest-driving, fastest-accelerating car that they can get. Today that means automatic transmission gas cars. If modern steam cars were available, I believe that they would eventually replace gas cars as the vehicle of choice, as they could beat the gas car in all of these consumer-important categories. And the steam car could also give lower emissions and better fuel economy on today's increasingly congested roads!


Fuel Mileage Of Actual Steam Cars

Demonstrating that properly-designed steam and gas cars get comparable fuel mileage, is the fact that current sport utility vehicles (SUVs), pickup trucks, and vans in the 5,000 pound weight range tend to get about the same 10-14 miles per gallon in typical city driving as steam-powered 1920s Stanleys and Dobles in the same 5000-lb weight range. Two examples are the 1999 Dodge Ram Sport (13 mpg EPA city) and the 1999 Ford F-150 Silverado (14 mpg EPA city). Owners of similar vehicles commonly report 10 mpg average in actual city driving. These vehicles have about the same weight and frontal area as Stanley steam cars of the 1920s, meaning that their air and tire drag, and therefore the engine horsepower developed to overcome these in typical driving, are (at first glance) comparable to those of the Stanley. And these 1920s Stanleys are reported by their modern drivers to get the same 10-14 miles per gallon in typical driving as their modern gas-engined equivalents.

Of course this is not an exact comparison. The differences, however, favor the steam car. Popular Science recently ran a chart showing that typical coefficients of aerodynamic drag of car bodies of the 1920s are about twice as high as those of modern cars. This means that for the same frontal area, a modern car has only half the air drag of a car of the 1920s. So the 1920s Stanley gets the same fuel mileage as a modern vehicle of the same weight and frontal area, while its engine is overcoming twice the air drag! Plus, for a given vehicle weight, the old-fashioned bias ply tires on a Stanley have higher rolling resistance than modern radial ply tires.

This suggests that the 1920s Stanley powerplant is more efficient than its modern gas car equivalent. If a 1920s Stanley powerplant were installed in a modern SUV, truck, or van, it would have less rolling resistance and air drag to overcome than in the original Stanley body of the same weight and frontal area. That means it would use less horsepower and therefore get better fuel mileage than the 10-14 mpg it got in its original (1920s) vehicle. Since the gas engine gets 10-14 mpg in the modern vehicle, and the Stanley powerplant would get better than 10-14 mpg in the modern vehicle, that means the Stanley powerplant would give better fuel mileage than the modern gas powerplant, with both powerplants installed in identical vehicles!

This also suggests that properly-designed steam powerplants would give better fuel mileage than gas engine powerplants in lighter vehicles, as well.

Of course, it is better to install a powerplant which keeps the proven design features of the classic steam cars, but which is updated for lower powerplant price, lower maintenance requirements, and greater operator convenience. Those are the design goals of the Brow steam car. I believe that these goals are achieveable, and could put the steam car ahead of the gas car in every department.





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