Gasoline is over $3.50 per gallon I have been noticing how few drivers seem to know how to drive more economically. People are accelerating, in the face of a red light and stopped cars; then breaking. I wonder if they know that it takes energy to accelerate a mass and decelerating converts that energy to heat in the break drums? This prompted me to write about the physics of internal combustion engines, and the effects of speed.
It seems few realize: at automobile speeds the aerodynamic drag increases with the square of the speed. At walking and bicycling speed the drag more or less increases linearly with speed, and at the faster speed of jet airplanes the drag goes up more like the cube of the speed. This means if you drive 80 MPH the energy you waste, just stirring the air, is four times as much as it would be if you were going 40 MPH. Simply slowing from 80 to 55 will more than cut the air drag in half. It won't double your mileage because other losses such as the friction in the engine and bearings don't decrease as much; never the less you will notice a significant increase in mileage. (A friend who has a Honda Incite says it changed his mileage from about 50 to 70 MPB when driving from San Jose to Los Angeles and back.) We can all get a similar increase simply by slowing down, and driving less aggressively.
Back in the late 1940's there was a club and to become a member you had to demonstrate, on a closed loop, you could get at least 80 Miles per gallon in a regular size automobile. How did they do that? There were several tricks to make your Studebaker or Chevrolet very efficient. They involved using a very thin oil in the transmission, differential and bearings; and over pressurizing the tires. But, the real trick was in the driving.
An engine that works on the Otto four stroke principle (and to a lesser extent a Diesel) is most efficient when it is going relatively slow with the throttle nearly wide opened. (I will explain why this is true a bit later.) To get 80 miles per gallon you have to take advantage of this fact. They used manual transmissions, and and would get into high gear as quickly as possible, usually around 10 to 15 MPH then accelerate up to about 30 MPH with the throttle held wide opened. Then take the car out of gear and turn the engine off and coast down to the 10 to 15 MPH, start the engine and put it in high gear and again accelerate and turn the engine off and coast again.
Honda and Toyota each have "hybrid" autos which get better gas mileage. Why, do they get better mileage? Regenerative breaking helps a little, but generating electricity and storing it in batteries is itself a pretty inefficient process. Indeed, generating electricity and storing it in batteries actually wastes energy. To explain why this is done requires a bit of understanding of the efficiency of a gasoline engine.
In an engine cylinder, the burning of the fuel significantly increases the temperature of the gas (air) above the piston. Some of the heat from the hot gases goes into the piston as it is pushed down. This cools the gas in the cylinder and it is put out through the exhaust valve at a lower temperature. The difference between the highest temperature and the exhaust temperature is directly a measure of the energy converted to mechanical energy. (Physicists know this as a Carnot Cycle. Named after: Nicolas L. S. Carnot 1796-1832) This was well understood by Rudolf Diesel. Diesel knew, if he could get the peak temperature higher and still put out the exhaust at about the same temperature he would have a more efficient engine. Introducing the fuel after the gas is compressed allows about twice the compression ratio. (Gasoline engines are limited to about 10:1 compression ratios while Diesels go to about 20:1) And we can see about 40% efficiency in optimal small diesel engines; nearly twice what an engine with spark plugs can get.
At best, a typical gasoline engine only gets about 20% of the energy from the gasoline into the crankshaft. Eighty percent goes out the radiator and tail pipe as heat.
Some very big diesel engines are getting near 50% efficiency, but they have cylinders over 3 feet in diameter! Increasing the diameter of a cylinder increases the area of the piston by the square of the diameter; but it only increases the cylinder wall proportional to the increase in diameter. For example: the piston top area of a 2 inch piston is one fourth that of a 4 inch piston. So a 4 inch piston could absorb four times as much energy as the 2 inch piston; but the drag of the piston rings on the cylinder walls would only be twice as much. Thus, the bigger cylinder makes the engine more efficient. Because of weigh and size, car manufactures are not making car engines with one big cylinder even though it would be more efficient. Also, they are selling to a public that believes "more is better", hence a 12 cylinder engine is thought to have some unknown virtue simply because 12 is a relatively large number.
The "hybrid" cars get much better mileage by taking advantage having a smaller engine which is more efficient because it has a better load factor. It's throttle is more nearly opened all the time. This gain is enough it can waste energy generating electricity, carrying and charging a battery; using the charged battery energy to get snappier performance than the smaller engine could provide. Even so, the hybrid car engines are still "too big" for best efficiency. It takes about 15 Horse Power, on the level, to move a Prius about 60 MPH in still air; but the Toyota has a 70 HP engine. The electric motors produce a bit more than 20 HP so working with the gas engine, they provide the performance the Toyota engineers felt drivers, and their advertising people would demand. I expect as the years go by and gasoline prices continue to go up, people will want/need to save fuel. When the market demands it, Toyota and other car manufactures will reduce the size of the engines to provide even better mileage, at the sacrifice of performance.
On the intake stroke the piston sucks the air/gas mixture into the cylinder. This takes energy from the piston. To slow the engine down there is a throttle plate that makes the piston work against a lower intake manifold pressure. If you only need 15 HP out of that 300 HP engine you could easily be using 45 HP creating the vacuum in the intake manifold; thus the engine is having to put out about 60 HP plus whatever is being lost to friction in the cylinder walls. Cadillac and a few others have tried just leaving the exhaust valve opened on some of the cylinders, so the working cylinders have a heaver load and hence working against less vacuum; but this does nothing to reduce the friction loss of the pistons against the cylinder walls.
Incidentally, a diesel engine also gains because it does not have a throttle plate, it simply injects less fuel. But, this introduces the complication of injection pumps, which is certainly justified for larger engines.
With newer techniques in the valve control, at lower power settings the engine can delay closing the intake valve letting the piston suck in more air/gas and then push it back into the intake manifold for part of the compression stroke; this part of the tricks used to get a little better efficiency. (I believe Toyota does this in the Prius. ??) One other trick is to not center the chrankshaft under the cylinders, so that when the cylinder fires the piston is pressing more straight down instead of pressing as much sideways. This reduces the drag on the cylinder wall on the power stroke, and boosts the efficiency a little more.
None of the above tricks accomplish nearly as much as simply putting in a smaller engine and operating it at a better load factor. Hence we are seeing the "hybrids" with all their batteries and complications.
Reducing the number of cylinders and making them larger would improve the efficiency. But, because of the size flywheel needed I doubt we will see a one cylinder automobile engine soon. But, they are used in some motorcycles. There are balancing and sound problems with a two cylinder engines. (Anyone who remembers to old John Deere two cylinder tractors knows the sound, which was not as "smooth" as most people want in cars.)
It will be interesting to see if an engine could be built with a "clutch" in the crankshaft so literally part of the engine could be stopped completely. Something like a four cylinder engine that could stop three of the four pistons and run as a one cylinder engine; until they were needed. I doubt this would be more complicated or heaver than having a generator, lots of batteries, controlling the charge of the batteries; all to have an electric motor that is only used once in a while?
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