I apologize for posting later than normal. Windows decided to perform an update when I sat down to finish this piece, so I lost around 45 minutes this evening.
The most important part of an automobile is how to power the wheels (or belt, if one talks of a snowmobile). What ever device does this must fulfill several requirements, which we shall look into later.
A device to propel a car must do several things, depending on the complexity of the automobile. It must produce enough power to overcome internal friction, to overcome air resistance, to overcome rolling resistance of the tires, and a couple of other things as well. Because of differences in combustion engines and battery operated motors, each type will be discussed separately.
Engines and Motors
These terms are often confused, used interchangeably, or otherwise misidentified. Strictly, an engine is a device that converts the energy in heat to mechanical motion by cycling between intake and exhaust conditions. An engine always uses a working fluid (a gas) that is at one temperature originally, then undergoes a compression/expansion (or the reverse) cycle to produce mechanical energy. They are governed by the laws of thermodynamics expressed in the Carnot cycle. One fundamental ramification is that an engine can only convert a finite fraction of the heat that is uses into mechanical energy. In the most basic terms, the temperature difference between the cold side and the hot side of the engine provides the fundamental limit of efficiency. Physicists, please do not criticize this oversimplified explanation. I just do not think that this is the venue to go into the Carnot cycle in depth.
In most automobiles, a gasoline engine uses the chemical energy stored in the fuel to produce heat, and that heat is used to move pistons to produce mechanical energy. Some use Diesel fuel, others propane, and still other specialized ones other fuels. The Wankel rotary engine does not have pistons, but rather a rotor that serves a similar purpose.
Motors, on the other hand, are not heat based. They are devices that use energy sources other than heat from a working fluid to produce mechanical energy. For example, an old fashioned water wheel driven mill that grinds grain is a motor, not an engine. Also, air driven tools like impact wrenches have motors in them, but not engines. Probably the most common motor is operated from electricity.
This might seem like splitting hairs, but the distinction is important. By the way, that idiom in French translates literately to “Cutting the hair in quarter.” I do not know why I thought of that. Unless you have the newest Nissan Leaf or a very, very old, turn of the 20th century battery powered car, your car has an engine, not a motor for its main source of mechanical energy.
Yet we still use the terms interchangeably. Even folks who know the difference always say “outboard motor” for a small engine that pushes boats, because “outboard engine” just does not fall on the ears correctly. Likewise, “motorcycle” is the term used rather than “enginecycle”. That is just semantics. As long as you remember that engines derive their energy from heat and that motors from sources other than heat, we will be fine for this discussion.
(Do not even ask me about a nuclear reactor, which is neither an engine nor a motor, but that is for a separate post. It is more engine than motor, but is different).
Let us assume that we are talking about a conventional, gasoline powered engine. It can have from one to however many pistons that are needed for the power requirements in the particular model. Lawn mowers, chain saws, outboard “motors” and the like often have only one cylinder, but can have more. In automobiles, common numbers are four, six, and eight, although I had a Geo Metro with three, and some very high end cars have 16.
Now, how does that engine work? To grossly oversimplify, let us assume a one cylinder engine like in a typical lawn mower. It works by taking air (oxygen) and gasoline, mixing them either by carburation or fuel injection, igniting the mixture to release heat, and having that heat drive a piston down with more energy than required to make the whole thing move. But it gets more complicated.
Most gasoline and Diesel engines are either two stroke or four stroke ones. (Some folks say two cycle and four cycle). In a two stroke engine, every other movement of the piston produces power, and in a four stroke one, every forth movement produces power, in theory, two stroke engines are more efficient since there is less wasted motion. For engineering reasons, four stroke engines are much (almost exclusively, as a matter of fact) more common in automobiles. Let us examine how they work.
All piston engines take advantage of the up and down motion of a piston, connected to a crankshaft and moving through a cylinder, to produce useful mechanical motion. Using a crank, it is possible to convert the up and down motion of the piston(s) to the rotational motion of the crank. But there are costs, because there is friction between the bearings every step of the way, and the piston rings also have friction.
Basically, a piston in a cylinder moves up to get ready to have the charge fired, then the charge is fired near the top of the travel of the piston, and the expanding combustion gases drive it down with considerable force, moving the crank to do useful work. Exactly how that is done differs betwixt two and four stroke engines.
It is actually easier to describe how four stroke engines work, since all of the essential functions are done one at a time. Once we understand them, two stroke engines will be easier to comprehend. There are two terms that need to be defined, first. One is Top Dead Center (TDC) and Bottom Dead Center (BDC). Those terms are actually pretty self explanatory, the first meaning the absolute top travel of the piston (and the throw (bearing surface) on the crank, by definition zero degrees), that the latter being the very bottom of the travel of the piston (and the lowest angular travel of the crank, 180 degrees).
Let us think about the workings of an engine. On a four stroke, we will arbitrarily consider the intake stroke first. In this instance, the piston is moving down the cylinder towards BDC. As it does, the volume of cylinder expands and at the same time, the intake valve(s) opens to allow a fresh supply of air and gasoline vapor (if a unit that uses a carburetor). Near BDC, the intake valve closes.
Then the crank pushes the piston towards TDC. This is the compression stroke. The gasoline and air mixture is compressed into a flammable mixture and gets ready to be ignited. In conventional gasoline engines, a spark plug uses and electrical discharge to ignite the mixture.
In fuel injected engines, most modern gasoline ones and all Diesel fuel ones, the fuel is injected into the top of the cylinder just before TDC, with no fuel/air mixture being taken into the cylinder in the first place. That is actually more efficient, and now only small engines use a carburetor to mix air with fuel vapor. In any event, near TDC, the spark plug fires, igniting the fuel and air mixture. In a Diesel engine, the heat high temperature from extremely high compression ignites the mixture without a spark. This is the compression stroke. All valves are closed during the compression stroke, obviously.
This causes the third cycle, the power stroke, to occur. The fuel and air mixture deflagrates and the energy released pushes the piston down, hard, imparting energy to the crank and keeping the cycles alive. I used the term deflagrate intentionally, because this explosion is controlled at subsonic speeds. If the mixture detonates (propagating at supersonic speeds) much damage is done to the engine. This is where octane rating comes into play, discussed later. All valves are closed here, as well, so as not to vent any useful expanded gases away from the top of the piston.
Just after BDC, as the piston starts to rise, the exhaust valve(s) opens, allowing the spent, burnt material to go towards the tailpipe. This is the exhaust stroke, and it completes the cycle. Then a new intake stroke starts, and everything repeats.
The engineering is sort of amazing. The valves have to hold a very tight seal during every cycle, and are usually equipped with very strong springs to keep them in place. A device called a camshaft is geared one way or another to the crank, and it mechanically lifts the valves at the proper time either to intake or exhaust. Often it is connected to the valve with valve lifters, and the modern ones work off of hydraulic pressure. It essential that the lifters operate at the proper time, or the engine will not be in sync. Older lifters were fully mechanical, and had to be adjusted by hand. Because they had only intermittent contact with the valves, and made a tapping sound, even modern hydraulic ones are still called tappets.
One of the reasons that there is a red line on the tachometer is that the mechanical connexions can not stay in time with the firing at extremely high speeds, so performance suffers. If you look at a typical horsepower curve you will see that there is a peak at a particular RPM, and that is falls off as rotational speed increases. There are a couple of other reasons, as well. Probably the most important is that as the rotational speed increases, the heat given off builds up before the cooling system can take it away. That causes the piston to expand, and so does the cylinder. The piston rings, the sealing parts, become unable to seal as efficiently betwixt the piston and the cylinder, and performance declines.
The most important reason for the red line is that lubrication is compromised at high RPMs. On most cars, there is an oil pump that is ganged to the crank in one way or another, and the rotational speed of the crank operates the oil pump. At extremely high speeds (and, thus high temperatures), motor oil tends to become less viscous (“thinner”) and does not cover the bearings as well. There is a point where the oil pump can not supply enough oil to critical parts and the engine fails.
On some extremely high performance automobiles, different methods are used to keep even thin oil on the critical parts. On modern Corvette engines, there are auxiliary pumps that collect oil during cornering and shoot it where it is needed, but this approach is obviously expensive. Extremely high performance engines also often use synthetic oils that are more stable from breakdown at high speeds, and thus, high temperatures.
But those are not the cars that we usually drive. Most of us have a relatively low tech unit that has changed little since 1940, except for the amenities.
Now we are ready to look into two stroke engines. These blend the four strokes into two, but they are more complicated to make work. The best of them are half again more efficient that four stroke ones, though.
In a two stroke engine, the exhaust stroke and the power stroke are merged. So are the intake and compression strokes. It is hard to do, but the results are good, for limited applications.
Let us consider a two stroke engine at BDC. It is at the bottom of the power stroke, and starts to rise. To run, it has to exhaust spent gases and take a fresh charge. Usually there is a hole in the cylinder and the gases pass though it into the tailpipe. But before that, it has to intake new fuel and air on the downstroke.
Most two cycle engines have a blower arrangement that introduces fresh fuel and air under pressure on the downstroke, and with a piston with a deflector on it to help keep that mixture inside the cylinder. As the piston rises, it forces out the rest of the spent mixture, and then covers the exit hole. Near TDC, the mixture ignites for the power stroke. As the piston falls, the pressure in the cylinder sucks in more fuel and air, and the cycle repeats. Because of the valving requirements, a two stroke engine is not twice as efficient than a four stroke one, but can come pretty close. But there extreme disadvantages.
For use in automobiles, two stroke engines have been pretty much dismal failures. Some large Diesel tractor trailer engines are two stroke, and they work well, as do some vary large fixed engines like on ocean vessels and in power plants. Most consumers are familiar with two stroke engines in small engines, like some lawnmowers, string trimmers, chain saws, and the like. Their high power output is well suited to these applications. The disadvantage is that the small engines are “total loss” lubrication. In other words, the engine oil is mixed with the fuel, either in the tank or by an oil injector and is burned during use. That makes them quite a bit more polluting than four stroke engines. In large two stroke engines it is possible to design systems that do not require the burning of the lubrication oil, so these are actually less polluting since they are more efficient.
I should point out there are some experimental six stroke engines that the very efficient, because they use some the the heat wasted by two and four stroke ones. This is beyond the scope of this discussion.
In an automobile engine, as the fuel and air mixture is compressed and is fired (usually a little before top dead center, since it takes time for the flame to develop), an explosion occurs in the cylinder, pushing the piston down. The relative volume of the cylinder between TDC and BDC is called the compression ratio, and the larger the compression ratio, the more efficient the engine. This is one of the reasons that Diesel engines are more efficient than gasoline engines, because Diesels have ratios of around 20:1, where gasoline engines are around half that. In gasoline engines, as the compression ratio increases, the gasoline and air mixture tends to detonate (the explosion propagates at supersonic speeds rather than subsonic speeds) and this is extremely damaging to engine parts, battering them.
The octane number of gasoline is a measure of its resistance to detonation. The higher the number, the more resistant it is. Detonation is called knocking or pinging by most drivers. Most normal performance engines run at a compression ratio of around 8:1, so 87 octane fuel is fine. Back in the day of the muscle cars, ratios of 11:1 or even higher were not uncommon, so 100 octane fuel was common, and even higher octane ratings are possible. In the old days, tetraethyl lead and ethylene bromide were added to raise octane ratings (if you want to know how that works, ask in a comment), hence the name “ethyl” gasoline. When EPA mandated removal of lead from on-road fuel, compression ratios were lowered and other materials used to boost the rating, ethanol being an excellent octane booster. Benzene, toluene, and the xylenes are also uses.
In a gasoline engine, a spark plug is used to ignite the fuel and air mixture, whilst in a Diesel engine the extremely high temperature reached by the high compression ratio is sufficient to ignite the mixture. In the old days, almost all gasoline engines were fitted with a carburetor, which mixed the fuel and air using the natural vacuum produced by the engine, and the engine intook the mixture with the gasoline in vapor form. (Except for the accelerator jets which squirted liquid gasoline into the intake manifold when the pedal was punched hard). Modern automobile engines now are almost all fuel injected, where a high pressure pump meters gasoline and squirts it either into the intake manifold (not the best method) or directly into each cylinder separately (the best method). Diesel engines are all direct injected.
Fuel injection is much more efficient than carburetors because the modern auto computer can control precisely the amount of fuel reaching the cylinders depending on load and speed with injection, while carburetors are approximate at best, and not amenable to computer control. In a modern car, the computer coordinates fuel intake, air intake, time of ignition (the precise position of ignition varies with load and speed, and generally needs to be earlier the faster the engine runs). In the old days, a centrifugal device was attached to the distributor and advance the timing with rotational speed. Computer control is much more precise. By the way, a modern car has many times more computing power than the Apollo spacecraft that carried us to the moon had. In a Diesel engine, the timing is determined by controlling the time where the fuel is injected into the hot, compressed air in the cylinder and there is no electrical component.
I think that we shall stop here for the evening, because there is no way that I can cover motors and keep the piece short enough not to be boring. We shall take them up in future. I think that next week we shall discuss the ignition systems used then and now, since getting ignition right is central to efficient running.
Well, you have done it again. You have wasted a perfectly good set of photons reading this low horsepower post. And even though Rand Paul (against whom I will have the pleasure of voting in November) stops to talk with reporters when he reads me say it, I always learn much more than I could ever hope to teach whilst writing this series, so please keep those comments, questions, corrections, and other thoughts coming. Remember, no scientific or technical issue is off topic here.
Warmest regards,
Doc
Crossposted at Dailykos.com and at Docudharma.com
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