Auto Emissions - physical realities

As the twin evils of over consumption of fossil fuel to power cars and the resulting emissions seem to be finally (after over 20 years of insanity) getting new attention, let us review a few inescapable facts surrounding automobile design.

More fuel is used during highway acceleration at any rate than while cruising down the highway at 110 km/hr. And the more vigorously one accelerates, the worst this is.

Why? Mainly because gasoline and diesel engines are horribly inefficient in cruise and much worse during acceleration. In cruise an engine delivers about 25% conversion to mechanical energy. That is to say that for every 100 litres of gasoline you buy, 75 litres is wasted as heat, never providing a benefit. As if that were not bad enough, it is even worse during acceleration, especially if the driver has a heavy foot. During acceleration all kinds of parsitic drag act on the engine. This includes induction air resistance (which the engine has to use more fuel to overcome), accessories (alternator, air conditioning, power steering, power brakes, etc.) and a richer mixture of fuel to air required when the engine is accelerating not to mention getting the exhaust out of the engine which adds a load just pushing it through the tailpipe (erroneously called 'backpressure'. It's simply resistance).

In effect, during highway cruise an automobile only needs a fraction of its installed horsepower. In turn it is wasting energy transporting that headroom horsepower around. Funny. (and wasteful).

It all comes to roost based on a very simple high school physics equation that is a result of the second law of motion from Sir Isaac Newton.

F = ma

Force, the 'strength' that something is pushed with is equal to the mass times the rate of acceleration.

Some people like to acclerate like crazy. 0 to 100 km/hr in 5 seconds! ("a" above). Well, to achieve that a powerful and high torque engine is required. (That's the "F" above).

Some people, often in the same group as above, also like their large vehicle and to have it lushly appointed for comfort and entertainment. That's the "m" above.

So, people want a lot of "a" and they like the comfort of "m". This means a big "F" under the hood that gobbles fuel whenever "a" is greater than 0.

Could it be worse? Well of course! In the real world there is no such thing as constant speed, at least as far as the engine is concerned. Huh? Well, it's like this: in the real world the auto faces wind resistance and rolling resistance. Wind resistance follows another equation derived from the equation above. I won't write it out here. Oh, okay, since you insist. It's:

Drag = Cd D S V^2 / 2

Where Cd is a constant representing the drag coefficient, D is the density of the air, S is the surface area of the car (seen from the front) and V is the speed (^2 means "squared").

When cars are less aerodynamic (as they've tended to become over the last 10 years) then "Cd" goes up. Cars have gotten large, so "S" goes up too. (Eg: a Jeep Cherokee is quite high compared to an automobile of the same width).

Drag goes up as a square of the velocity. There is 4 times as much aerodynamic drag at 100 km/hr than at 50 km/hr. Compound that with poor aerodynamics and large area and the Drag is really high.

Back to the top. Drag means that if you take your foot off the gas, the car will slow down. When at constant speed on the highway, your foot is always on the gas. The power of the engine is working against the deceleration (-a) due to the wind and rolling resistance. The engine is always applying a force to overcome the force of drag. It is a hidden "F=ma" if you will. Funny how that crops up to use more gas.

Clearly, other than reducing speed (say 100 instead of 120, you do the math, remember to square) a large improvement can come from improving the aerodynamics of the car, esp. by reducing the frontal area of the car and making the rear tapered so it doesn't pull a roiling mass of air behind it (notice that airliners have pointy tails?).

Next, consider rolling resistance. This is the tires against the road. Tires are soft compared to the road so energy is lost in continuously reshaping the tire as it rolls. (This is why the temperature of the tires rises as you drive). The faster you go, the more resistance. The softer the tire, the more resistance (as there is more deformation/reformation as well as more tire in contact with the road.) The larger the car, the larger the tires and even more is in contact with the road. (Trains are about 3x more fuel efficient than trucks in part because they have very high wheel pressure on the rail)

This is where things compound and make cars horribly inefficient.

As vehicles get larger they obviously get heavier. As cars get heavier, they need larger engines. Larger engines mean larger transmissions, drive trains and support structures. Larger cars have larger wheels, larger tires and larger brakes.

All of those "largers" also mean more weight. More "m" from the first equation above. And to make it worse, we seem to want a lot of "a" meaning bigger engines and the consequences of that.

Clearly it is past time to put childish things asside.

Smaller cars with moderate acceleration requirements mean a lot less surface area to resist the wind, a lot less rolling resistance against the road, a lot less weight to accelerate and stop and smaller engines that require less fuel to accelerate the above. It is a compounding effect.

The wrong use of recent engine efficiencies.

Ironically, engine efficiency has improved remarkably over the last 30 years. Electronic fuel injection, microprocessors, variable valve timing, aluminum block engines, advanced induction and exhaust systems and more have combined to get more power out of the same sized engine. However, while the power to weight ratio has improved, the consumption per hp has not improved as much.

Car makers have taken advantage of the higher power to weight ratios to push around heavier vehicles even while giving them the "desired" performance in acceleration.

One has to ask the question: since power to weight ratios have improved, why not keep power constant and make the engines smaller, lighter and cheaper? That would result in improved fuel efficiencies as it would contribute to lighter transmission and driver train, smaller wheels and brakes. Instead, led by the former "big 3", Mercedez and BMW, automobiles post records for more and more power every year with scant (if any) improvements in fuel efficiency.

Turbochargers.

Turbochargers could help, however they are most efficient as a function of exhaust gas flowing through them (called volumetric efficiency). This means that they are very good at increasing the horsepower of an engine, but that is only a benefit during acceleration. And during an acceleration the turbocharger only helps the engine use more fuel. (Note that on piston powered aircraft turbochargers have only a beneficial effect: they allow operation at high altitude where the density of the air is lower, hence less drag [same equation above applies]).

Next: let's discuss electric vehicles for Canada. It's not a clear cut case.