How a Car Works

That Magic Pedal…

I’m talking about the gas pedal, of course. As far as many people know, it’s just magic – when you push down on the pedal on the right, the car goes faster. Well, sorry to crush that childhood illusion. Unlike Santa and the Easter Bunny, it’s all a big lie. It isn’t magic at all. In this section I’ll lay out the basics of how your drivetrain shoves ‘Ol Betsy down the road, and turns irreplaceable fossil fuel into forward progress.

Most modern cars have piston-type internal combustion engines. They most commonly have between three and ten cylinders of varying size, but let’s talk about a four cylinder engine to illustrate some operating principles that apply to most current production cars. Refer to the pictures whenever I confuse you. Each piston travels up and down in a hole in the engine block, which is usually made of cast iron or aluminum. The piston has a connecting rod attached to it that is called, oddly enough, a connecting rod. The other end of the connecting rod is attached to a crank. Think of pedaling a bicycle. The piston is your knee, the connecting rod is the lower part of your leg (the part of your leg that contains the tibia and fibula), and the pedal on the bike with the little arm is the crank. Up above the top of the piston is what’s called the cylinder head, which contains the intake and exhaust valves. As the names imply, the intake valves let air into the cylinder, and the exhaust valves let the exhaust, or burned fuel, out. When the intake valves are letting fresh air in, gas is shot into the area as well. After the valve closes, the spark plug ignites the air and fuel mixture, forcing the piston downward, like your knee driving the bike pedal down. When the crank pushes the piston back up, the exhaust valve opens to let the burned gases out. Pushing harder on the magic pedal increases the amount of fuel and air that goes into the engine, increasing its speed and power.

Just a quick comment on the smorgasbord of devices for sale on TV, in auto parts stores, and even in your local Walmart that claim to increase gas mileage. I’m sure those little magnets to put on your fuel line, or the gizmo that goes on the throttle body are going to double your mileage. Heck, if you install enough of these magical devices, your engine will probably pump fuel into the gas tank! The car manufacturers have some fairly competent engineers working for them, and they have almost unlimited resources available to them. They also have a ton of pressure to increase fuel efficiency both from the government and from consumers. If any of that hocus-pocus junk worked, the manufacturers would be installing it at the factory. These products are really only designed for one single purpose – to relieve you of your hard-earned cash. Don’t be fooled. The best way to optimize fuel mileage is to take care of your car and drive gently.

All this activity, with the friction and the combustion, generates heat, of course, and the cooling system is in charge of keeping the engine at the correct temperature. As everyone knows, overheating is bad – but running too cold is also not good. Coolant, or anti-freeze, is circulated around through passages in the engine. It’s then pumped to the radiator, which cools it, then it’s pumped back into the engine for another round trip. Anti-freeze is also used to heat the passenger area. There’s a little mini-radiator in the dash that handles that chore. The engine is internally lubricated by a pump that circulates oil through the engine in a manner similar to the coolant. The engine oil gets pumped around inside the engine, passing through a filter at some point, and then drains into the oil pan at the bottom of the engine, where it waits for another round trip.

The fuel delivery and spark delivery systems are controlled and monitored by the car’s computer. The computer also monitors, but does not control, many of the purely mechanical activities going on in the engine (and also in the rest of the drivetrain). The reason for monitoring activities that it can’t control is so that the computer can alert the driver if something in these systems isn’t working right. It typically does so by turning on the Check Engine light; for more info check out my section on that.

So, we have the crank spinning merrily away – how do we harness that spinning motion to move a car as we would have it moved? We stick a transaxle between the engine and the drive wheels. The word “transaxle” is a combination of two other words – transmission and axle. What this means is that a single assembly does the work of a transmission and the work of an axle, or differential. Let’s say our imaginary car has a manual transmission. When you push the clutch pedal in, the spinning crank is disconnected from the transmission, allowing the car to be stationary while the engine is running. When the clutch pedal is released, the crank is connected to the trans- mission’s input shaft, so if the transmission is in any gear (not neutral), the car must be moving if the engine is running. The transmission is equipped with a variety of gear ratios to suit whatever speed (within reason!) the driver wishes. The output of the transmission is delivered to the wheels of a front-wheel-drive car by the drive axles, which have special joints in them to allow for the motion of the suspension and steering. Automatic transmissions work in a very similar way, with the shifting and the clutching/declutching being controlled by elec- tronic and hydraulic systems.

Modern drivetrains are very reliable, and only require routine maintenance to deliver hundreds of thousands of trouble-free miles. Regular fluid flushes, oil changes, filter replacement, and, at longer intervals, timing belts and tune-ups will insure getting the most out of your engine and transmission.

As your car’s primary driver, you should be alert to anything out of the ordinary that might be the symptom of a problem. Unusual noises, roughness, leaking fluid, loss of power or gas mileage, or a check engine light are some things that should prompt a visit to your mechanic for a check up. We have some forms that our clients use to compile information that might be valuable for a technician trying to sort out a problem. These forms also facilitate communication between you and your mechanic, and help to eliminate ambiguity.

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