Ronin of the Spirit

Because reality is beautiful.

Great Fuel Economy without a Big Oil Conspiracy or Hybrid.

“Why won’t the car companies build this car?”

I can’t tell you how often I have heard that from people in reference to some high mileage concept.  Well, luckily you all have me here to answer that for you.  Today we are are going to make an high fuel economy car on paper, then I’ll explain why no one builds it.

There is one simple way to use less fuel: make less power.  Engines burn fuel to make heat, then convert this heat into horsepower.  To save fuel, we need to make as little heat as possible, then convert that heat into power as efficiently as possible, then take that horsepower and use it to move the car as efficiently as possible.

There are 2 ways to reduce the need for power: Aerodynamics and Weight.  We turn to the Power Train (engine and transmission) to convert the energy efficiently from heat, to horsepower, to vehicle motion.

Aerodynamics.

Aerodynamics are simple.  The faster the car goes, the harder the air in front of it piles up, sticks to the sides and swirls around behind it.  Step one to good aerodynamics is to make the car’s cross section as small as possible.  No matter how aerodynamic something is, the bigger it is the more air it has to push out of the way.  So, make the car very narrow and low, say 44″ wide and 44″ tall.  (Airplanes made to sit two across are this size.  Its doable, just different.)  To keep the air from piling up in front, the nose of the car needs to be a rounded point like a bullet.  To keep the air from swirling around in back in needs to end in sharp point, like a wedge, and should be quite long.  Since some air sticks to the sides, the longer the car, the more air sticks.  If the car is too long more energy is lost unsticking the air from the sides, than swirling around behind a blunt a wedge.  6 times the length is ideal for a wedge.   The car would be about 23 feet long, but we can cut off the last 3 feet to make a “Kammback” and have it be just as good.  The car then ends in a straight edge, which is good for mounting the tail lights in anyway.

Weight

Weight is also simple.  The more it weighs, the more power is needed to accelerate, climb hills, and stop.  The last is important for two reasons. One, heavy cars need heavy brakes.  Heavy brakes mean a heavier car, which needs a heavier engine to get around, which in turn becomes heavier and needs heavier brakes. (Don’t laugh, this is why  a 73 Corvette weighs 500 lbs more than a 53 Corvette.) Two, among existing mass produced cars there is proportional relationship between weight and and likelihood of the passengers to survive a crash.  There are ways around this, but it requires some real design skills.  Bearing safety in mind, we want the car as light as it can be inexpensively made.  The only option this really leaves us is an aluminum space frame with a lightweight plastic body covering it.

Power Train

Engine

Power Train includes the engine and transmission.  We need to use as little fuel as possible.  Hybrids sip fuel by using a battery pack and electric motor to move the car at low speed and the engine to move it at full speed.  The problem is that the very best, cost-no-object batteries still don’t even hold a 1/10 the energy per pound as tank of gas.  So we will hybrid with a small engine, say 5 to 10 hp.  This engine will run the A/C and anything else necessary when the car is stopped, help accelerate it at low speed, and let the primary engine take over at higher speed.  Since the secondary engine is so small, and used occasionally, it doesn’t need the special “getting the most heat out of the engine” trick that the primary engine does.  To accomplish this we need a something called a “turbo-compound engine“.  I’ll not explain the intricacies of these here, only to say it involves a turbo that uses some of its power to supercharge the engine (like a normal turbo) and returns further power to the crankshaft.  The maximum efficiency for this set up is about 60% vs the 20% most cars make.  However, it is unlikely that in vehicle service we could get over 40-50% efficiency.  Basically double.

Transmission

The car is very light, but people aren’t.  So the car might only have to carry its own weigh plus a 160 lb person, or four 200 lb people and some luggage (a 1000 lbs).  This means the load range of the car is 625%.  To pull this off we need an unusually flexible and efficient transmission.  Luckily for these relatively low loads, there is an ideal one which shifts without gears, called a Continuously Variable Transmission or CVT.

The technology

So what went into the car?  The chassis is a welded and bonded aluminum space frame, covered in plastic panels. The Renault Sport Spider does this, and its chassis weighs less than 180 lbs.  The primary transmission is an of-the-self CVT unit, but the car needs three additional transmissions.  One to connect the secondary engine to the primary transmission, one to connect the secondary engine to all the auxiliaries of the primary engine, and one to connect the turbo to the primary engine. The secondary engine is a standard 100cc motorcycle engine.  The primary engine, on the other hand is a direct injection, turbo-compounded unit.  Though this is old technology and regularly used in power plants and other other very large engines, no one has made any transportation engines of this type since the Wright R-3350 of the 1940’s and 50’s.

Proformance

The car should have at least twice the aerodynamic efficiency of a normal car, so that doubles the mileage once.  The engine should produce its power with half the fuel of a normal engine of the same size, so double again.  Going with the mileage of existing economy cars, the Ford Festiva and Geo Metro, 40-50 MPG and taking it times 4 we get 160-200 MPG highway.  Using the example of modified economy specials from the 70’s (which never went over 30 mph) we can estimate the in town mileage of around 300 – 400 mpg.

Production

Space frame chassis do not translate well into mass production.  The more purely the form is a space frame rather than a unibody, the more this is true. (Saturn’s “space frame” chassis aren’t really.) They must be semi-mass produced, which raises the price.  The power train can be mass produced, but requires premium components in many places to function.  It also has four transmissions.  So, again the power train is expensive.  If the car is going to sell for a reasonable price, these expenses must be made up in the only remaining ways: body, non-critical component quality, interior trim quality, and lack of amenities.

Body: Instead of being the shiny, ultrahard plastics that Saturns are made of, it will be the cheap matte injection molded plastic that storage tubs are made of, and the even cheaper diecut plastic that notebooks are covered with.  The windows will be fixed, and bonded to the body.

Non-critical component quality.  This means parts that work in a way that makes you nervous.  Door handles that flex horribly before opening, blinkers that stay on until you shut them off manually, and gauges will be plain digital readouts, as if robbed from a microwave.

Interior trim quality: This mean lawn chair like seats, and and lack of fascias.  The guts of the dash will be just sitting there.  No head liner on the ceiling, just bare plastic.  No carpet or rubber mats on the floor, just bare metal. Or, conversely, if the fascias are installed, they will be of cheap material and installed sloppily.

Lack of amenities: No power steering, windows, brakes, seats, mirrors, locks.  Nothing is powered at all. No stereo, no GPS, no gear shift (push button for forward and reverse)  Spartan, spare, and minimalist.

The whole picture

So now we have our super mileage car.  It gets 300 MPG in town and 200 MPG on the road.  It costs about as much as a normal car, it comes in one color, a sort of beige gray (the cheapest plastic), and it is shaped like a turd.  You can’t use drivethru’s anymore because the wheels stick out a foot from the car and the windows are fixed in place.  You are as safe in a crash as anyone else in accident in a small car, but thats not saying a lot. You can carry 4 people and all their stuff across the country on 10 gallons of gas.

Answering, “Why don’t they make it?”

Well, quite simply, the lead times and costs are enormous.  I would buy this car because I would rather get 300 mpg than look cool.  However, most people would rather have a much more compromised car which gets 40 mpg instead of 30, and is a better phallic extension for them.  There simply aren’t enough people who would buy these to justify building a factory to produce them.  Besides, the kind of people who are so cheap they will drive what looks like a wheeled suppository just to save some scratch aren’t going to buy a new one every 5 years.  They are going to keep it like an heirloom.  Which means there is no continuing demand. Once everybody who wants one has one, they can’t sell anymore.

Finally, every company has a culture. It is no more acceptable in Detroit to be really excited about building a super economy car than it is for a school teacher to be really excited about taking preschoolers to the bathroom.  Oh sure, both parties will do the job because it is their civic virtue, but both would be highly suspected of aberrant desires if they were really excited about it.

Car companies are not in the business of selling transportation machines.  They are in the business of selling desire.  There is no profit margin on utility.  A car you actually need would probably cost about 5 grand, look at the Tato Nano.  The only way for the car companies to make that additional 25,000 dollars is to sell you what you want instead of what you need.  Do people want to get 200 mpg gallon?  Certainly, but not nearly as bad as they want to look the part of whatever dream they are having.  Men and woman who have never even seen a gravel road buy off road packages because it compliments who they like to see themselves as.  The number of people who want to look in the mirror and an ecologist more than they want to see a sexpot is just too slim to make a car for them.

July 29, 2008 Posted by | Ecology, Government, Microcar, Small Car, Uncategorized | , , , , , | Leave a comment

Sterling Engine Analysis

In 1816, the Reverend Robert Stirling invented a engine. From time to time an astute reader will hear about this engine as the solution to the world’s problems in general, and as the perfect candidate for automotive hybrids specifically. Pure bunk and here’s why.

First, you have to know a little about hot gas. (Gas like air or CO2, not like gasoline.) When gas is heated, it wants to get bigger (ie. increase in volume). When it has its heat removed (cooled), it wants to decrease in volume. If it is in a sealed container, it can’t increase its volume, so it presses against the walls of the container all the harder when heated (pressure). That’s why aerosol cans say to not expose to temperatures of more than 120 degrees. They are full of gas at a certain pressure and if they get too hot, the pressure gets too high and they pop. (Also, if you have a weak container, like an empty closed pop bottle and stick it in the freezer, it will collapse. The removal of the gas’s heat causes a reduction in volume, which reduces the pressure, meaning the air pressure on the outside is higher, and it squishes in.) There are mathematical formulas that describe these relationships of pressure, volume and temperature called gas laws.

Nice suit!

A man named Carnot (above) put all of the gas laws together and drew some rational conclusions. He performed a thought experiment about the perfect heat engine. (An engine being a device for turning heat energy into mechanical energy.) The perfect heat engine would be made out of a magic material which would let heat in but not out at one point, and out but not in at another. It would have no friction, and would never leak. That way, any energy flow could be controlled and monitored.

(1.) Heat would be added perfectly instep with the expansion of the gas, so that no energy was wasted. (Heat is added but the temperature doesn’t increase, because it’s expanding instep.) It expands while taking heat, pushing the piston down.

(2.)The heat in the gas is then “used up” as the gas continues to expand without new heat. It expands while cooling, still pushing the piston down.

(3.)The heat is then removed from the gas, causing the gas to shrink (reduce in volume), pulling the piston in.

(4.) Now the piston is pushed in (further reduced in volume), raising the temperature of the gas back to the temperature it was before the heat was added in step 1.

For various reasons deduced from the gas laws, Carnot’s engine is the most efficient on earth. Since we know what perfect is, we know the best way to design any engine on earth.

Though every part of Carnot’s cycle is right, none of them are true, and therein lies the problem. There is no material which can conduct heat only in one direction, give us choice of direction, and switch direction at whim. There is no gas which behaves exactly as the gas laws say they should, though hydrogen approaches it. There is no material that is totally frictionless and perfectly sealing at the same time. Carnot’s engines says the key to efficiency is the difference between the temperature of the heat input in step (1) and the heat removed in step (2).

Material science is the kicker. The engine’s material must be a good conductor of heat or the heat in it will build up until it melts. But it must not be too good a conductor of heat or it will take heat out of the engine which the engine is supposed to be making power out of. It must allow a tight seal for the piston but without to much friction.

Long story short, the engine must be all at once: a good conductor, insulator, bearing surface, and pressure vessel. Due to the properties of combustion, it must do all of this while white hot and resistant to corrosion.

In the case of the Otto engine (the kind most likely in your car), you can add to all of those challenges this: the heat is not made outside the engine, but in it, and the gas not heated by an outside source, but within the cylinder itself by flame. Furthermore, there are the complexities of piping the gas in and out.

At this point one might cry, “Wait a moment! Do you mean to tell me that the efficiency of an engine is based of the difference between the temperatures at the beginning and end of the cycle? My exhaust manifold GLOWS red! I must be throwing away a huge amount of energy!” Yup.

And that’s a very good thing. The reason you can afford a car is because the Otto cycle, with all its oddities of valves and spark plugs and not reusing the working gas, dumps excess heat out the exhaust stream. If it didn’t, the engine block would need to be made of the same alloys that jet engines are made of, instead of cast aluminum or cast iron.

So, here’s the danger of a little information. (And full circle back to the Stirling engine.) The Stirling engine does not burn inside the engine, it burns outside of it. Its gas is sealed away inside. Of all the engines in the world, Sterling comes the closest to Carnot’s imaginary engine in its cycle. Only in its cycle. Remember that Carnot’s engine is imaginary and made of unobtanium? Carnot’s cycle only has meaning as a thought experiment because you can’t make an engine out of magic alloys which do not exist.

People read that the Stirling engine is theoretically the most efficient heat engine and assume they don’t have one under their hood because it was simply never maximized. Actually it is not that it has not been maximized but that it CANNOT be maximized. Though combustion creates temperatures of thousands of degrees, the Otto engine need not operate at that temperature. If a Stirling was going to operate at that temperature the heat would have to move through the engine and then into the gas. So the engine block, under full power generating stress, must be hotter than the low stress exhaust pipes of an Otto cycle.

Though invented in 1816 to save people from the danger of boiler explosions, the Stirling was never widely used. Steam engines are also external combustion engines, but they have the boiling of water to serve as a temperature regulator. Stirlings do not have this, and a frequent and persistent complaint is burnt out parts.

Another HUGE misunderstanding about Stirling engines is their ability to use very low temperature differentials; that is to say, freakishly small differences between input temperature and output temperature. It’s true. In a 72 degree room, a small Stirling can run off the heat of your palm. These tiny engines create just enough power to overcome their own friction. What would happen if you scaled it up? You would have an enormous engine with equally enormous bearings. Again, the engine would create just enough power to overcome its own friction.

But just for the sake of argument, let’s say you had a truly enormous engine, one the size of a house. The hot part is in the sunshine, and the cool part is in the shade. The low temperature difference would be overcome by the truly enormous amount of energy available, right?

A qualified no. The smaller the temperature difference, the greater amount of gas the engine has to pump around to get the same amount of power. There’s no free lunch. For the same amount of power, high temp = small working mass, low temp = large working gas. The losses to pumping all that gas through the small passages necessary for heat reclaiming mount up very quickly. For this reason, efficiencies are very low. While low efficiencies with free power (like solar) are OK, it’s a niche application.

Another route to efficiency is high pressure. Reverend Robert made his Stirlings low pressure and large (For instance, about a cubic foot of displacement per horsepower, or 172,800% larger than an Otto cycle of the same HP.). The modern trend is to make them high pressure and small. But then they must be filled with inert gas and sealed just so, because if air and lube oil are pressurized and heated the Stirling engine becomes a bomb. This is also why Rev. Stirling could make his engines with a foundryman and bricklayer and modern engines are “lab queens” in college physics departments.

Finally, another story that pops up now and then is Ford’s Stirling research in the 1970’s. Yes, they made a Stirling engine. No, they didn’t produce it. They didn’t produce it for the exact same reason Chrysler didn’t produce its turbines nor GM its Wankle. Material science could not mass produce certain key components at low enough cost to get enough people buying. This, in turn, means mass production could not be used, further raising the price and decreasing the market in a vicious catch-22.

Don’t get me wrong. I think Stirlings are cool. I think they have applications to green science. But we will never see one in a car produced by market forces. Further, if you want to invest in expensive technologies, fuel cells have higher real world efficiencies than Stirling’s theoretical ones.

April 9, 2008 Posted by | Ecology, Engines, skepticism, Small Car, Uncategorized | , , , , , , , , , , , , , , , , , , , , , , , , , | 4 Comments