Engine Requirements:  Automakers must design engines under a long list of requirements.  Among these are requirements for:

- All weather operation and all temperature operation

- Drivability

- Low levels of pollution & unwanted emissions

- Safety (mechanical & fire safety)

- Adequate power for accelerating, passing, towing and climbing hills

- Acceptable fuel economy

- Size; the engine must fit within the car

- Longevity (the engine must last at least as long as the other car components)

- Ease of service and maintenance

- Acceptable levels of vibration, noise, and minimal odors

As you can see, good fuel economy is just one of the requirements levied on automakers by the public or the market.  How does the public do this?  By buying things they like and not buying things they dont, basically.  There are also restrictions imposed by the law, such as for safety and acceptable levels of exhaust emissions.  And don't forget that design changes can also be caused by the legal industry.  As a result of lawsuits, manufacturers will adapt what they do to avoid legal troubles.  That goes for all industry, and the automakers are certainly included in that.

Key Point #5:  Car engines must satisfy many requirements, and good fuel economy is just one of about a dozen.

Now you know that the problem of designing engines for cars isnt simple, and I havent even discussed engine internals and functionality.  Before I do that, I will present some sections on fundamentals.

Combustion Basics

We live in air, and need air to breathe.  Put very simply, air gets mixed in our bloodstream and provides the oxygen that mixes with the fuel we eat, called carbohydrates, to keep us going.  Air is also used by car engines, to mix with hydrocarbons, to keep them going.  We exhale carbon dioxide and water and excrete the unusable liquid and solid matter.  Cars also exhale carbon dioxide and water and some excess particulates and gases.  The similarities are not a coincidence.  Fuel for cars and fuel for humans both contain similar elements, but their molecules are organized in a different way.

Air and Oxygen

Recall that air is made up of 78% nitrogen.  The rest is almost all oxygen, which is about 21%.  The last 1% is a mixture of all sorts of other gases.   The oxygen is important because without it, fuel wont be processed in our bodies or burned in car engines.  Steam engines used oxygen from the air to burn wood in the furnace, which made hot steam; the steam moved big pistons to power the old steam engines.  Jet airplanes breathe air to get the oxygen they need to mix with jet fuel (jet fuel is a thick gasoline, kind of a gas-diesel mix).  Rockets take oxygen with them.  After all, there is no air in space; without air, there is no oxygen, so rockets must prepackage fuel and oxygen in tanks; the fuel and oxidizer (with oxygen in it) are mixed to create combustion to power the rocket.

Cars, though, just use oxygen right out of the air.  Air burns with fuel almost perfectly when mixed at a ratio of 14.7 pounds of air (not gallons, but pounds of air) to 1 pound of gasoline (about 2 cups).  A fuel-injected engine mixes air and gas at that exact ratio pretty much all the time.  Unwanted emissions are minimized when this ratio is maintained.  If the engine received a fuel-air mixture of 13.5 air to gas, it would be rich with gas, meaning there would be insufficient oxygen to burn the gas.  Unburned gas would come out of the tailpipe as a smelly black smoke.  Driving behind one of these cars is unpleasant, because of the smell that goes right into your windows or through the fresh air vents located near your windshield wipers.  

An engine that burns lean puts more air into the combustion chambers than the gas needs to burn, creating excess heat, and results in more NO and NO2 emissions.  Chemically speaking, there are not enough hydrocarbons in a lean combustion process to provide Carbon atoms (the C).  As a result, the excess oxygen finds singled out Ns that got broken apart because of the extreme heat.  The O and Ns form NO and NO2 which contribute to smog and acid rain.

Now I will move on to engines.  I will start your understanding of engine designs with a brief description of engine internal parts and describe how they work together.  Then I will give a top-level chronology of American automobile engines.  This will make it easy to understand what has happened with the industry.

Engine Fundamentals

Engines are often described as air pumps.  Air is drawn into the engines combustion chamber by the piston moving down and creating a partial vacuum condition.  While the air flows through the intake manifold into the combustion chamber, fuel is sucked in along with it (with a carburetor) or sprayed in (fuel injection). 

The piston moves back up and compresses the fuel air mixture and at that instant, the spark plug fires and the mixture burns, creating hot gas in such a large quantity that it shoves the piston back down, which turns the crankshaft.  The crankshaft is bolted to the transmission, which is connected to the drive axle and the wheels are bolted to the axles.  The engine turns, the transmission turns, and so on, until the car is moving.

Back in the engine, the piston travels back up, it pushes out the gas that is now all burned up.  At the top again, the burned gas has rushed out and the piston goes down again, once more drawing fresh air and gasoline into the cylinder.  The pistons are all sequenced to suck in fuel and fire one after another, so that while some are just sucking in air and gas into their cylinders, others are firing. 

The doors that let the intake air into the combustion chamber are called valves.  The valves are actuated (opened and closed) by a mechanical device called a camshaft.  The longer the camshaft keeps a valve open, the more air and gas it lets into the combustion chambers of each cylinder, and the more power the engine will make.  Engine makers can change how much power an engine makes by altering how large the valves are and how long the camshaft will keep those valves open. 

Many modern engines are multi-valve engines, meaning they have more than the traditional one intake and one exhaust valve.  The multiple valves allow more gas and air to come into the combustion chamber with the same, low-lift, short duration camshaft.  That camshaft is typically good at producing its maximum torque at low engine speeds.  So, the engine is now capable of good torque at low, medium and high speeds.

The output of an engine is measured in horsepower and torque.  Torque is the rotational output energy; when you multiply torque by an engine speed multiplier, you can calculate horsepower.  The rotational energy directly results from the spinning crankshaft that is made to rotate by the powerful up and down motion of the pistons.

Engines Summarized

Automobile engines are essentially air pumps.  They suck in air, mix it with gas, and compress this mixture.  Then a spark sets off the mixture, which burns very fast and hot, resulting in a very large amount of hot gas.  The piston was pushed down by the expanding, burning fuel & air, and as it travels up, it rushes out the exhaust valve.  The pistons up and down motions are turned into rotating torque via the crankshaft.

If you increase the compression of an engine, it will make more torque and power.  If the valves, intake and exhaust ports and throttle body are larger to match a higher lift or longer duration camshaft, the engine will make even more power.

Engine Effects on Fuel Economy

Now that you understand how engines work and how they can be modified to make more torque and power, I will present what this means to our original problem:  fuel economy.

Simple engines with two valves per cylinder, that have low lift, short duration camshafts with correspondingly small ports and valves can be matched with drivetrain gears that keep the car revving at low engine speeds.  These configurations were the industry standard for many years.  Because the engines were only efficient at lower engine speeds, highway driving was not very fuel efficient, regardless of how aerodynamic the car was.  If the engine was large, such as a big V-8, it probably got bad city mileage too.  But, if one of these traditional low-tech engines, in four cylinder form, was put in a modern, lightweight, aerodynamic car, it would get very good mileage.  Nowadays, this isnt done.  But in the recent past, it was and this resulted in savings because the automaker would not have to spend much money to develop a fancy engine.  That savings could be passed on to the customer by a low price for the car.  Examples of low-tech, low-revving engines in cars are:

·         1985-87 Pontiac Grand Am (2.5L 4-cylinder), 34 mpg highway

·         1988-92 Chevrolet Camaro (5.0L V-8), 27-28 mpg highway

The downside to such cars is lower performance.  The Pontiac above did not accelerate well compared to newer cars, and the emissions were not in the ULEV (Ultra Low Emissions Vehicle) category with new cars.  But, I argue that achieving ULEV or SULEV status does not by itself compromise fuel economy.  Achieving more complete combustion should result in two things:  Creation of more power and  less undesirable emissions.  The Camaro performed reasonably well, but GM moved on to more modern engine designs anyway. 

Modern engines use higher compression ratios, modern camshaft designs to limit the amount of unburned mixture that can get pushed out through the exhaust valve by the rising piston, and modern combustion chamber designs.  Therefore, the modern engine should be better at everything; it should get better mileage, make more power over a wider range of engine speeds, produce less undesirable emissions, and be more reliable to boot.

Some of these engines do just that.  The most recent generation of the GM 3.8L V-6 engine meets this criterion.  So does the Oldsmobile 3.5L V-6, which is perhaps the most impressive engine to come out of Detroit in the last 20 years.  This is an excellent engine, giving much greater torque than its European and Japanese competitors, with excellent power at high rpm all while giving the Oldsmobile Intrigue mileage that compares with the Honda Accord V-6 and bettering the 3.5L Nissan Altima.  Equally as impressive is the LS1 V-8 in the 1997-2003 Camaros and Corvettes.  These engines give the cars they end up in some pretty impressive mileage to accompany the improved acceleration.  The 1999 Camaro Z-28, with this engine and an automatic transmission is rated at 27 mpg on the freeway, better mileage than a 1996 Honda Prelude, a very aerodynamic, smaller, and lighter 4-cylinder car with a manual transmission!

Modern multi-valve engines are getting to be more common.  These engines are in many of the newer cars.  The benefit of this technology is that the engines can rev much higher because they flow so much air with the same, conservative camshaft profiles that two valve engines use.  This way, camshaft overlap is still minimized, but the engine gets much more air inside the chambers because of the multiple valves.  Therefore emissions are still low, and the power band of the engine is wider, meaning that decent torque is created from low RPM to very high RPM.  A good example of a car with a modern multi-valve engine is the Honda Accord.  The 1999 Accord with a 4-cylinder variable valve engine and automatic was rated at 22 mpg in the city and 30 on the highway.  For comparison, a 1999 Oldsmobile 88 with the larger 3.8L V6 engine was rated at 19 and 29 mpg.  Now this is a much heavier car, with a larger, more powerful engine.  The Oldsmobile engine has only two valves per cylinder.  Not high-tech on paper but clearly doing the job very well.

Key Point #6:  Many modern cars with multi-valve engines achieve less fuel economy than cars equipped with two-valve per cylinder engines, even if the cars are lighter and smaller and equally as aerodynamic.

Here are additional examples.  For further comparison, refer to the examples above.

·         2003 Mitsubishi Eclipse, 4-cylinder, 16 valve engine, automatic transmission, weight empty: 2,910 lbs:  Fuel economy rating: 21 mpg city / 28 mpg highway

·         2003 Lexus IS 300, 3.0 V6, 24 valve engine, automatic transmission, weight empty 3,200 lbs.  Fuel economy rating:  18 city / 24 highway

·         2003 Chrysler Concorde, V6, 12 valve engine, automatic transmission, weight empty 3,480 lbs:  Fuel economy rating: 21 city / 29 highway

·         2003 Pontiac Bonneville, V6, 12 valve engine, automatic transmission, weight empty 3600 lbs:  Fuel economy rating: 20 city / 29 highway

In the example above, we see that the first two cars are multi-valve engine equipped and the bottom two are much heavier, larger cars with bigger engines with more power.  It's interesting to note that the US-made vehicles move more weight, have to push more air out of the way, and do better on a gallon of gas. 

The real disappointment is the Lexus IS300, which competes directly with BMW's 3-series car.  The 24 mpg rating on the highway is lower than in similar cars.  When this car was introduced, it had a highway rating even lower, although the larger V8-powered LS400 was rated at 25 mpg on the highway.  This is incredible! Of course the LS400s rating of 25 mpg isn't great but it beats its own brand's little brother and does so with more weight and a larger, more powerful V8 engine.  Typically, increasing weight and size isn't in the formula to improving fuel economy!