The art of engineering for high fuel mileage

If only the rumors, claims and stories were true, we all could be realizing triple the mileage we currently experience. High mileage carburetors, hydrogen injection, magnets, any number of high mileage promises and conspiracy theories simply don’t deliver on their claims. I have tried and/or investigated them all.

Actually, there is enough energy in a gallon of gas to go 200 miles, if the engine was 100% efficient. But it is not, nor can it even come close.

vapor engine


Increased mileage comes from hard dedicated work, advanced engineering, money and a lot of design and prototype hours.

Let’s discuss the possibilities.

Aerodynamics and rolling resistance are important considerations, and I have incorporated improvements in these categories in some of my high efficiency automobile prototypes, but, for this article I am going to focus on the engine and transmission, the two components that hold the secrets to real improved fuel mileage.

The greater potential of the two is the engine.

The best gas engines return around 30% efficiency. The industry claims that for every gallon of gas, 25 to 30% converts to actual power, 30% is lost to the combustion chamber cooling via the radiator, and around 30% is discharged via the exhaust. The remaining few percent is due to mechanical losses. My own tests are close to industry claims, but not the same. I have tested actual horsepower requirements via test instruments for 3 vehicles, a Honda Civic, a Dodge Neon, and a Ford Focus, all vehicles are about 3000 pounds and all required around 14 horsepower at a steady 55 MPH. but the reality is that to compensate for all average road conditions, the ups and downs, the stop and starts, it requires around 25 HP as an average and around 150 HP as a total.

A gallon of gas contains about 125,000 btu’s (British thermal units, a measure of energy). Let’s assume a 3000 lb. vehicle averages 30MPG and we run it at 60 MPH for one hour (highway), that’s 2 gallons of fuel or 250,000 btu’s. 250,000 btu’s equates to 96 HP for one hour at 100% efficiency, but we know we only consumed an average of 16 HP for an hour not 96 HP. That is an efficiency of 16% not 25 or 30 as industry claims. (If the car was consuming 24 HP as an average, than it would be 25% efficient).

Obviously, the trick to higher engine efficiency is to shift the percentage numbers to favor higher power output per unit of fuel by reducing heat loss, so let’s look at specific heat loss categories.

The exhaust heat is simply what is left over after the power stroke, and some efficiency work can be done, but the heat loss (and therefore power loss) due to cylinder quenching is the target of my efforts. When a particular cylinder fires, there is over 2000 degrees(more or less depending on compression ratio and throttle position) of hot gasses expanding into the cylinder where the hot gasses pushing the piston down come in contact with the cylinder walls that are around 250 degrees. This is called cylinder quenching, and the proportionally cool cylinder walls and head absorb heat (and therefore power) from the hot expanding gasses reducing the cylinder pressure. If this heat loss to the cylinder walls, piston and head could be minimized, there is significant engine efficiency to be gained. Because that heat was not absorbed by the cylinder walls, the exhaust temperature is proportionally increased to the point that it can be converted to additional power, and there are a couple of ways to capture the exhaust heat.

1.) Turbocharging
2.) Using the heat to change a liquid to a gas (steam) under pressure and using the pressure to drive a turbine, piston or vane engine.

There are also several ways to reduce the power loss from cylinder quenching.

1.) Cylinder quenching minimized by the use of ceramics to insulate the combustion chamber:
The use of ceramics has been an ongoing effort by several automobile companies for several years. Even Smokey Yunick was engaged by automobile companies to try to develop a dependable ceramic or ceramic coated combustion chamber, but with little success.

2.) Cylinder quenching minimized by reducing or eliminating the combustion chamber cooling system:
Eliminating the engine cooling system is a way of reducing heat(power) loss via cylinder quenching and is a trick utilized by high mileage competitions that colleges participate in and was one of the main strategies used by Shell for their famous high mileage car in 1973 (along with a form of fuel vaporization). Unfortunately inadequate lubrication from sustained high combustion chamber surface temperatures, metal fatigue from high temperature, incoming intake air being preheated from high temperatures and simple impracticality, all lead to a dead end for the engine that has to produce sustainable and dependable horsepower.

3.) Shorter intake stroke compared to the power stroke:
If we take a normal nominal 10:1 compression ratio engine, normal 14.7 psia atmospheric pressure, allow the atmospheric pressure to enter the engine during the intake stroke, and then compress the air 10:1 during the compression stroke, we would ideally see 147 psia (10 times atmospheric pressure) on the compression gauge plus/minus pressure variation due to compression heat and blow by for a gauge pressure of 125 – 135 psig. For ease of math between psi actual and psi gauge, I will use gauge pressure so let’s assume we are at 125 psig (gauge). When the engine fires, the cylinder pressure rises 2 to 4 times depending on throttle position for a typical firing pressure of 250 to 500 psi. If we use 300 psi as normal cruising cylinder pressure we would need a cylinder 3 times longer than the intake stroke or 2 times longer if we consider heat loss to the combustion chamber surfaces (cylinder quenching) to bring the cylinder pressure back to where we started (14.7 psi). However, it is not practical to bring the cylinder pressure entirely back to atmospheric (because it is primarily the residual power stroke pressure that pushes the exhaust gasses out, not the exhaust stroke) so ideally we would want an engine with an intake stroke of say, 3 inches and a power stroke of maybe 5 inches. That is what the Atkinson cycle engine accomplishes mechanically, but the complexity of the mechanism makes it impractical. Today’s Hybrid and some non-hybrid engines use a technique where the intake valve is held open a calculated distance past bottom dead center to effectively shorten the intake stroke compared to the power stroke.

4.) Combustion chamber quenching minimized by changing combustion chamber volume as a ratio to combustion surface area:
For instance, if you have an engine with a 4 inch bore and a 3 inch stroke, you have 37.62 cubic inches of cylinder volume. In the same cylinder, the surface area is 62.8 square inches. Therefore the cylinder volume to cylinder surface ratio is .599. Let’s hypothesize that this is a 4 cylinder engine, so we have a 150.48 cu. in. engine. (The cylinder to surface ratio remains the same regardless of the number of cylinders) This means that for every cu. in. of cylinder volume, there is about 2 square inches of combustion chamber surface sucking up the heat (power) during the power stroke. Now let’s imagine a single cylinder engine of approximately the same as the 4 cylinder with dimensions of 6.125 inch bore and a 5 inch stroke. That comes out to 153.3 of volume and 159.449 of combustion chamber surface area for a single cylinder volume to surface area ratio of .96, almost 1 to 1. This means that for every cubic inch of cylinder volume in this engine there is only about 1 square inch of combustion chamber surface, half of what we had in the 4 cylinder engine example. This results in less heat (power) transfer to the cooling system during the power stroke and a higher exhaust temperature after the power stroke.
This approach holds significant opportunity for fuel savings.

5.) Combustion chamber quenching minimization via high speed power stroke:
If we think about a 4 stroke engine, there is cumulatively 15 minutes of intake, 15 minutes of compression, 15 minutes of power and 15 minutes of exhaust for every hour the engine operates. This means that for every hour the engine runs, there is 15 minutes during the power cycles of 2000 degree hot gasses exposed to the 250 degree combustion chamber surfaces, resulting in the cooling and condensing of the power stroke gasses during the power stroke. If we could, for example, double the speed of only the power stroke as compared to the other strokes, the hot gas to combustion wall exposure time would be cut in half, resulting in less hot gas quenching and therefore increased pressure during the power cycle.

    Our engine has design characteristics designed into the power stroke mechanisum that allows for a quicker power stroke, and therefore the term pulse engine.

6.) Variable combustion chamber space:
There are circumstances where smaller or larger than average combustion space is warranted. For instance, if a low horsepower need is at hand, a small combustion chamber combined with a matched starved intake will yield low horsepower with high efficiency. If high horsepower is a requirement, a larger than normal combustion chamber is dialed in along with some turbo boost and we have high horsepower (see pulse engine development chronology HERE).

Transmissions hold the next best chance for improved gas mileage

The function of the transmission is to effectively connect the power of the engine to the road. The more ratios available in the transmission the more efficiently the transmission can do its job. Today 6,7, or even 8 speed transmissions are the norm along with efficient variable speed drive transmissions (see IVSD chronology page HERE).