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Aug 30 2010

Pique the Geek 20100829: Automobiles, Part III. How to Keep Everything Going at Once

We have talked about how engines work in the past couple of installments of this series, and now need to bring together some important engineering factors.  We have talked about intake strokes, compression strokes, and others, but only very superficially touched on how everything comes together.

For an engine to work properly, everything has to be coordinated.  For simplicity, we will consider a conventional four stroke, gasoline powered automobile engine.  Diesel engines are in some aspects simpler, and will be covered concomitantly.

Remember, an engine has to do all of the things about which we have discussed, completely synchronized, and EVERY time.  For an engine rotating at, say, 4000 times per minute, this can be a daunting task.

To review, let us remember that there are three critical events that have to occur properly to make the engine described above “go”.  First, the pistons have to move in a predictable manner for the four strokes.  Second, the valves have to open and close in a predictable, synchronized manner with the pistons.  Third, the source of ignition has to occur at just the right time, also synchronized with the movement of the valves and pistons.  Forth, for fuel injected engines, the introduction of fuel has to come at the right moment and be synchronized with the rest of the operations.

The pistons are attached to the crankshaft via push rods connecting rods (often just called rods) with bearing surfaces on both the piston end and the crankshaft end.  Oil is essential to keep them lubricated or else they will seize under load, but that is another story.

The valves, however, are NOT directly attached to the crankshaft, yet have to operate as if they were for everything to work at the proper time.  They have to open, stay open for a definite amount of rotation of the crank, and then close properly.

One the intake stroke, the piston moves down, making the volume in the cylinder larger, and thus producing a partial vacuum.  It is essential for the intake valve to open then, to allow air only (in a fuel injected engine) or and air and fuel mixture (in a carburetor equipped engine) to enter the combustion chamber.  This is done by a camshaft, which is somehow “tied” to the crankshaft, either by gears, a chain, or a belt.  These diverse methods aim for the same purpose:  to get the camshaft to rotate in communion with the crankshaft, over and over.

The camshaft is also borne on bearings that are lubricated with oil, and has some mechanism to keep it right with the crankshaft, as just said.  The camshaft has lobes that are shaped to open and close the valves at the proper times.  Those lobes are usually pretty much ellipses with one end (on the shaft itself) large and round, tapering off to a rather delicate, polished end that works the valve itself.  On most modern automobiles, because of the high stresses, the lobes operate valve lifters that are almost always hydraulically coupled to absorb stress betwixt the lobe of the cam and the stem of the valve.  Otherwise, wear is a real problem.  The valves are contained in the cylinder head on most modern engines.  In the old days the valves were in the engine block itself.  The valves are connected to the head by retainers and strong springs to keep them firmly closed until the valve lifter mechanically pushes it open.  By the way, valve lifter is a reference to the old days when valves opened “up”.  In overhead cam engines, valves open “down” since they are upside down relative to in block valves, but the term stuck.

Now, the cam has two lobes (at least) for each cylinder, because there are two (at least) valves per cylinder, an intake and an exhaust one.  On the intake stroke, one lobe of the cam opens the valve connected with the air supply, allowing air (or air and fuel) to be drawn into the cylinder.  Near BDC, the intake valve closes because the cam lobe is no longer in contact with the valve lifter as the cam rotates.  (If you are not hip with engines, it is important for you to go back and read the previous installments in this series to understand).  Now, the crank pushes the piston UP towards TDC with both the intake and exhaust valves closed, to allow the charge of fuel and air to be compressed.  (There is a slight difference for directly ported fuel injection, and we shall discuss that in a little bit).  Just before TDC, for reasons mentioned in past installments, the spark plug fires, igniting the fuel and air mixture.  For injected engines, the fuel is also introduced into the cylinder just before then, and for Diesel ones just at the moment of firing.

The ignition causes the piston to pushed DOWN the cylinder, and the energy from this ignition provides the energy for the power stroke.  It makes sense that no expanding gases escape during this time, so both valves are still closed tightly, just like in the compression stroke.  Near BDC, the camshaft’s other lobe, machined to contact the lifter that opens the exhaust valve, causes that valve to open as the piston rises.  That arrangement allows the spent gases to be pushed out of the cylinder.  Thus, in a typical engine, for every one cylinder that rides on a throw from the crank, there are two lobes on the camshaft that operates the valves.  Some engines have multiple valves per cylinder, so the cams are more complex, but the concept is the same.

Now we have integrated the piston action with the valve action, but we have left out when the spark plug fires (or on a Diesel engine, since they have no spark plugs, when the fuel is injected).  On gasoline engines from the 1920s to very recently, a mechanical distributor was used to coordinate when the spark plug fires with the other activities in the engine.  This device was almost always geared to the crankshaft, and operated mechanically. Its design was circular, with electrical contacts, the number of which depending on how many cylinders the engine had, contained in the rotor cap.  For a V-8 engine, there were eight contacts, at regular intervals, six for a six-cylinder engine, and thus.  My Geo had only three contacts, because it had only three cylinders, plus one contact no matter how many cylinders to allow the current into the distributor.  The rest feed the current to the spark plugs.

The distributor operated by allowing a high voltage electrical impulse to be directed to a particular spark plug in a given cylinder depending on the position of the piston in the desired cylinder.  It was powered by a coil that was always “hot”.  In other words, the coil stepped up battery power (either six or, later, 12 volts) up to around 35,000 volts or so, enough to make a spark betwixt the electrodes of the spark plugs.  Distributors worked mechanically, and the making and breaking of the DC circuit in the points induced a high voltage in the coil that was “distributed” to the particular cylinder (spark plug) that required it.  The points were mechnically activated electrical contacts, similar in function to a light switch on your wall.  This was all coordinated by gearing and geometry.  Because of the high voltages involved, the electrical contact mechanism (the points) eroded rapidly, even though platinum was often used to resist that.  A condenser was usually added to the circuit to reduce the impulse and spark at the points to make them last longer.  That required adjustments to make everything work on time, so the dwell was important.  The distributor actually has a tiny cam on it to open and close the points, and by slight movement of the of the contacting mechanism on the point set, the points could be held open for a greater or lesser amount of time.  That is what the dwell is:  the amount of time that points dwelled open or closed.  In addition, by loosening a retaining nut, the entire distributor could be rotated to advance or retard the spark plug firing with respect to the position of the piston.

So now we see that everything is either geared, chained, or belted together, and any deviation from this close dance will throw off the timing of the engine, with deleterious results.  But it that still is not so simple.

As engines are “revved”, it becomes necessary to ignite the spark more and and more before TDC, because of the lag in ignition of the air and fuel mixture and the complete combustion of it.  The speed of the explosion does not change, but the speed of the moving parts do, and this relative difference must have compenstation.  Thus, the distributor had to have some mechanism to make it fire more and more BTDC to keep the engine running right.  Most cars from the 1920s until the mid 1970s used a centrifugal device to advance the timing, because as the weights were thrown out by centrifugal force, the distributor was “twisted” sort of like loosening the retaining nut as mentioned before.  The rotation of the distributor shaft caused a rotating contact (often called a bug) to make contact from time to time with the contacts leading to the spark plugs.

I should talk about cranking a Model “T” Ford by hand.  There was a mechanical spark advance (timing from ATDC to BTDC) on the steering column.  To crank on by hand, you had to push the spark advance all the way to the upper stop, or you would break your arm.  Here is the logic:  If the engine starts ATDC, it continues on its merry way, round and round, but sputteringly.  If you start it BTDC, it may well turn the compression stroke to a power stroke before its time, and the impulse is sent to the crank that you are holding in your hand.  However, the engine would rev and would not run for long with the spark advance retarded.  Thus, you would crank it until it just started, then run to the steering column, and pull down the lever to advance the ignition timing.  Then she would settle down and run.

I compromised.  I used my foot with a few clicks of the advance lever down and just essentially kick started it.  With nothing for it to kick back into and injure, I could start the car with the first kick almost always, and did not have to hurry to advance the ignition.  But that was growing up learning to drive on a Model “T”.

Now everything is different, although the cams and the crank are still geared, chained, or belted together.  Very few modern production cars have a mechanical distributor, but rather an electronic module that, when ganged together with a position sensor on the crank, does the work that the distributor used to do, but much more efficiently.

In modern cars, the coil is replaced by a solid state, electronic module that collects data from the crank position, the rpms of the engine, and the power requirements to “know” when to fire the spark plug.  Oxygen sensors also contribute data to the computer to allow the engine to run at maximum efficiency, at least for regular cars.  It is possible to buy “chipping” darkware that alters the computer software to make the car perform better (insofar as acceleration and top speed goes) with the sacrifice of fuel economy.  That is almost like “camming” and older car and changing the timing advance.

Now the electrical timing is computer controlled, as is the injection of fuel for direct injection engines.  The less efficient indirectly injected ones work like the ones with a carburetor, except that squirts of liquid fuel hit an impinger and is then sucked into the intake manifold, much like the older technology.  The advantage is that the computer can meter the fuel, even in one of these, much better than the old technology ever could.

Since Diesel engines have no spark plugs (for reasons elaborated on in earlier installments), the ignition timing is governed by when the fuel is injected into the cylinder, because it ignites spontaneously because of the extremely high temperatures.  Thus, electricity is not directly involved in Diesel ignition, except for control electronics.  Thus, they never had distributors or electronic ignition modules.

As you can see, that are lots of things that have to happen at just the right time, over and over, often at thousands of times per minute, for an engine to work properly.  If any of the parts mentioned fail, then performance and economy suffer, or the engine may even fail to run at all.  In modern engines, most of which use timing belts to connect the cam to the crank, an even more serious thing can happen.  Timing gears and chains rarely fail catastrophically, but timing belts can, and do, break.  Gears and chains wear or stretch, causing performance to decrease, but on SOME engines if a timing belt break under load, severe engine damage can occur.

Think back to the valves and pistons.  In normal service, as the engine is near TDC, all valves are closed.  If a timing belt break (I am using the subjunctive mood of the verb “to break”, and the lack of a terminal “s” is intentional), and the camshaft be in such a position as to hold a valve open as the piston approaches TDC, it is possible for the piston to make contact with the protruding valve head, bending the valve shaft, breaking the piston, and doing other unpleasant mischief.  Not all engines have this problem, but some popular ones do.  I will leave it for the mechanics in the readership to point out the ones bad about this.

Finally, let us thing about the change in the meaning of “routine maintenance” of engines with modern (since around 1975, give or take a few years) and ones from the earlier years.  I have a 1998 Ford Windstar and a 1967 Chevrolet Camaro.  The Windstar is a V-6 and Camaro a V-8.  Both of them require regular oil changes, and the requirements for those have not changed much over the years.  By the way, the single most important thing that you can do to make your engine last is to change both your oil and filter (not changing the filter leaves dirty oil in the engine) on a regular basis.  We can discuss the frequency of that further in the comments if you wish.  Tire pressure, chassis lubrication, and other mundane operations are also similar.

Here is the big difference.  On the Windstar, I have to replace the spark plugs around every 90,000 miles or so.  The plug wires should be changed around every other plug replacement.  Other than the occasional timing belt, that is it.  Period.

In the Camaro, the spark plugs, points, condenser, rotor, and rotor cap have to be changed around every 12,000 miles, the timing and dwell reset, and the wires around every 60,000 miles.  In addition, the carburetor has to be adjusted periodically.  Older cars require much, more maintenance than modern ones.

Most of that is due to the modern, high energy, electronic ignition.  The old mechanical systems physically wore fast, and the low voltage spark wore the spark plugs fast.  The points would pit and fry (in a pinch, you could file them and reset the dwell to get a few more miles out of them) and had to be replaced.  The bug and distributor cap would arc and become poorly conductive, so they had to be replaced then as well.  With ignition voltages pushing 100,000 volts, spark plugs last much, much longer now.  That sounds sort of counterintuitive, but for the low voltage systems the gap between the electrodes on the spark plugs was normally around 0.035 inches, and on modern ones around 0.080 inches.  The hotter spark reduces fouling of plugs and for some reason erosion of the electrodes as well.  Better materials are also responsible, I am sure.

In contrast with an engine, an electric motor is simplicity itself.  In future installments we shall discuss these in more detail.

Well, you have done it again!  You have wasted a many einsteins of perfectly good photons reading this low octane post.  And even though Glen Beck admits that is really all about him when he reads me say it, I always learn much more than I could ever hope to teach by writing this series, so please keep questions, comments, corrections, recollections, and other communication coming in the comments.  Remember, no technical or scientific subject is ever off topic here.

Warmest regards,

Doc

Crossposted at Docudharma.com and at Dailykos.com

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