mailto:dave@lafn.org More transportation articles by David LawyerCopyright 2008 by David S. Lawyer. Feel free to make copies but commercial use of it is prohibited. For example, you can't (except to an insignificant degree) combine it with advertising on the Internet. Please let me know of any errors or suggestions for improvement.
Most people erroneously think that to save gas they should take it easy on the gas pedal, accelerate slowly, and maintain steady speed. But in reality one should do just the opposite combined with lots of coasting.
To drive efficiently, keep a heavy foot on the gas but when you wind up going too fast, shift to neutral and coast. It would be even better if you could coast with the engine off but in most cases it may hurt your transmission. And it usually disables the power part of the brakes and steering which may not be safe. The brakes and steering still work but you have a use a lot more muscle to operate them. Except if you should have a hybrid, it's designed to coast with the engine off.
Also, coast downhill as much as possible and also coast when you need to reduce speed or stop. On a level road you can "pulse and glide" which is done by accelerating at about 2/3 of maximum acceleration (pulse), and then when you reach a high speed, shift into neutral and coast (glide) until speed is reduced by say 15 mi/hr. Then repeat another pulse and glide and so on.
It takes skill to do this safely and in heavy traffic it may be nearly impossible. Make sure that you are not obstructing traffic behind you. Note that coasting in neutral is illegal in many states but shouldn't be. In some states only coasting downhill in neutral is illegal.
The reason that the above works is that while automobile engines are about 30% efficient (thermal efficiency), the typical efficiency while driving is more like about 12%. The efficiencies at various RPMs and torques may be found by looking at the "brake specific fuel consumption map" for your engine. Looking at this map shows that engine efficiencies range from 0% to 30% and that the best efficiency (30%) is only obtained at a certain RPM and torque. By using this map you can find the most efficient torque for the RPM your engine is at and apply that torque (using the gas pedal). This will provide the maximum efficiency for that RPM. Of course the car's computer could assist in all this if only the car makers wanted it to.
While doing this will get you 30% only at a certain RPM (like 2,500) at other RPMs efficiency doesn't drop off that much except at very high RPMs. For example, any RPM below say 3,500 might give say at least 28% efficiency provided you also provide the right torque with gas pedal. Wind the engine up to 4,000 RPM and the maximum efficiency may drop to 26%. And higher rpm's will be significantly worse. This is all still a lot better than the typical 12% people get out of their engines while cruising at moderate speeds (at low torque).
Where do you find a "brake specific fuel consumption map" (BSFC map) for your engine? Good question. Car manufacturers keep them secret and don't distribute them although it's possible that some auto-makers don't even have them. Of course you could buy an engine, put it on a dynamometer test stand and test it yourself at various speeds and torques and create your own map. Or you could try to get one from an automotive engineer that was involved in engine performance testing, especially from one that no longer works there. Or you could try to get laws passed to require release of such information although such a law may be unconstitutional. Or you could try to get the EPA to come up with these maps when vehicles are tested for fuel economy.
Even if you had such a map, how far do you press the gas pedal down to get the optimal torque? For this you need a torque meter on the dash, but they don't seem to be available. However, not all is lost since the optimal points are all at fairly high torque. One problem is that you do not want an automatic transmission to automatically downshift since then RPMs will then be high and high RPMs give lower fuel economy. Also, if it downshifts, then it has to upshift again and all this causes needless wear on your transmission.
So selecting a torque at moderate speeds is fairly easy. Press on the gas pedal so that the transmission is just on a verge of down-shifting, but don't push it far enough to actually downshift. At non-moderate speeds (high and low speeds) press the gas pedal so that the acceleration is about 2/3 of the maximum that would be obtained without downshifting. At low speed, downshifting is not so detrimental since the engine speed will likely not be too excessive even after it downshifts. At high speed (if you're lucky) automatic downshifting may not happen if you keep torque under 2/3 of the maximum.
The methods for fuel economy described above are quite simple and easily understood. One operates the auto engine at near its maximum efficiency (at high torque) and stores the excess energy produced in the momentum of the auto for later use while coasting. This momentum represents "kinetic energy". In the case of coasting downhill, energy is stored when the auto ascends a hill and gains elevation. This stored energy is known as "potential energy" and is expended when coasting downhill.
A hybrid auto operates in a similar way by storing energy in an electric storage battery. It saves the excess energy by generating electricity in a generator, storing the energy in a battery and then later withdrawing the energy from the battery to power the auto. This process is by no means 100% efficient like kinetic energy storage is: There are losses in each stage of the power flow: generator-battery-electricMotor that are not present in either the kinetic (or potential) energy storage schemes. That's why some hybrid owners don't use the hybrid in it's normal mode of battery charging/discharging, but pulse-and-glide instead since it's more efficient. But with the hybrid it's at least feasible to coast with the engine off resulting in a more efficient pulse-and-glide.
Pulse and glide is also good for road sections other than level. For uphill sections, the pulse will be longer and the glide shorter. If the uphill grade is really steep, there may be no glide phase: It will all be powered climbing and you might want to do this when the glides become too short on an upgrade. For downhill, the pulses will be shorter and the glides longer. And if the downgrades are moderately steep, it will be all coasting with no power from the engine needed. If they are still steeper you may wind up braking with the engine with the transmission in gear.
For going over the crest of a hill where engine power is not needed for going down the other side, try to go over the top of the hill at a slower speed since the downgrade itself will speed up the auto from the slower speed of going over the crest. This may mean coasting just a bit as you approach the crest of a hill.
Since hills can be used to store energy, it turns out that driving in hilly areas may result in higher miles-per-gallon, provided of course that the downgrades are not steep enough to require braking. In fact, even without any coasting in neutral and pulse/glide, fuel economy on a mountain (or hilly) road (with the start and end of the run at the same altitude) should be better than on the level since even if one doesn't coast in neutral but lets off on the gas pedal, much of the potential energy is still recovered on the downhill sections.
The main problems with coasting are: laws, failure to design autos for coasting, and the effects on other traffic. The last problem may improve as the amount of driving is reduced due to both higher oil prices and possible population decline due to a poor economy, etc.
The need for changes in the laws speaks for itself. It might be feasible to have a pulse and glide lane where all cars in this lane would synchronize their pulse and gliding, possibly by electronics or colored lights on the roadway.
But major changes are needed in the design of autos to support coasting. First, it should be possible to prevent automatic downshifting of automatic transmissions when the driver doesn't wants it. Autos should be able to coast with the engine off which means electric power steering and possibly manual hydraulic brakes. A super-high overdrive gear ratio would help too, allowing cruise at high speed with the engine at slow RPM with high torque for economy.
One reason why high torque is more efficient is because with higher power output (due to higher torque), the engine parasitic losses: friction, pump, and alternator power are a lower percentage of the useful output power. It takes fuel to just turn the engine when it's not supplying any output power. This "parasitic" power is used to overcome friction in the sliding parts of the engine such as the pistons, to power the alternator, and to power the following pumps: water, oil, power steering, air conditioning. The energy used by an idling engine is all parasitic: The power produced is all used just to keep the engine turning and no power (or torque) is transmitted to the wheels. Thus the efficiency is zero.
There are actually 2 types of power produced by the engine: brake power and indicated power. The power on the rotating output shaft of the engine is the "brake" power. If you put a brake on the output shaft and braked the engine (at constant speed (like driving with the brakes applied), the power dissipated by the brake would be the "brake" power. Now the use of the word "brake" in "brake specific fuel consumption map" should be clear. Unless otherwise specified, engine power and torque, etc. mean brake power and brake torque.
The power produced by the pushing down of the pistons by the exploding fuel mixture (assuming the pistons had no friction with the cylinder walls) is "indicated" power. It called "indicated" since if you put a pressure gauge on each cylinder, read a pressure indicator as the piston goes down on the power stokes, and calculated the work done by this pressure, then the power of this work would be the indicated power. It's pretty obvious that to find indicated power in practice you would use a computer to read the pressure gages, rpm, and make various calculations, etc. As indicated power goes up, so does fuel consumption.
Now imagine that you are driving with the engine engaged but not supplying any torque to the wheels since you are essentially coasting. This might be the case if you were driving down a moderate downgrade. The engine is essentially idling, but at high speed. Your engine is producing indicated power but no brake power. If you press down on the gas just a little more, then your engine might use 1% more indicated power, most of which would be applied to moving the car. The result would be that about 99% of the indicated power is still being used for parasitic purposes: friction, pumps, alternator, and only 1% is being used for moving the car and producing output torque. Even if the indicated power was being produced with 35% thermal efficiency, the brake efficiency would only be 1% of this or 0.35% (nearly zero). This example shows one reason why internal combustion engines are inefficient at low torque.
There's another reason too. When the engine has low torque it also has high vacuum due to the throttle value restricting air flow into the engine. In contrast, at high torque with the gas pedal pushed nearly to the floor, it has low vacuum. High vacuum means that the engine is like a vacuum pump, expending energy to draw air into the cylinders (pumping losses). So low torque implies high vacuum which implies high pumping losses which implies lower efficiency.
Thus as torque approaches zero, so does engine efficiency. That's why efficient driving needs high torque and a heavy foot on the gas pedal.
If by pulse and glide I could double thermal efficiency of the engine from say 12% to 24%, why doesn't my miles per gallon double?
One reason is that you may not be applying enough torque during pulse. And if you do apply sufficient torque, your transmission may downshift which greatly increases the RPMs. Now high RPMs not only reduce the engine efficiency but accumulate angular momentum in the engine/transmission which is wasted energy. Of course, since you have neither a torque gauge nor a BSFC map, you don't know exactly how much torque should be applied (how far to press down of the gas pedal) for maximum efficiency.
Another problem is that while you're coasting, the engine is likely idling, thus consuming energy when the car isn't using any energy. Also, there is more aerodynamic drag using pulse and glide. If you coast from 70 mi/hr down to 50 mi/hr the aerodynamic drag, which is directly proportional to the square of the velocity, will be greater than if you just maintained a steady 60 mi/hr.
Due to traffic and slow cars ahead, you can't pulse and glide is subject to interference making it less efficient. Also in urban driving a high percentage of energy is wasted in braking and much of this is hard to avoid. So even if cruising efficiency doubles using pulse-and glide but half the energy of normal driving is wasted in braking, then miles-per-gallon would only go up 50% instead of double.
Thus while pulse-and-glide might double your mi/gal under ideal conditions, under real condition you are lucky if it increases it by say 40% or so.