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Tuesday, June 14, 2016

Four stroke engines - basics

On the earlier post about Two-Stroke Engines, much of the focus was on the difficulty two-strokes have with exhaust gas and intake charge flow reversal and flow-through.  There was also some discussion of engineering techniques that attempt to overcome these issues.

Four-stroke engines don't suffer quite so much from these type of problems, because in a four-stroke, there are four distinct processes, which for the most part are segregated from one another.  Additionally, in modern engines, valve timing and lift can be adjusted while the engine is running.

See the animation below for how air flow works in a four-stroke engine.

  1. Intake:  The cam at the top right pushes down on the inlet valve, just as the piston reaches the top of its travel.  The piston creates a suction as it moves back down, drawing in a combustible fuel/air mixture.
  2. Compression:  At the bottom of the piston travel, both intake and exhaust valves are shut.  They close because they are fitted with springs to shut them when the cam lobe is not pushing down on the valve stem.  The piston rises, compressing the fuel/air mixture.
  3. Power:  At the top of piston travel, a sparkplug ignites the fuel/air mixture, causing it to expand rapidly, forcing the piston downwards, creating power at the engine crankshaft (bottom).
  4. Exhaust:  At the bottom of piston travel, the top left cam opens the exhaust valve, and the piston travels upwards, expelling the spent exhaust gas, at which point step 1 begins anew.
The engine above would be considered to have a modern valve arrangement: The Dual Overhead Camshaft (DOHC).  There are two cams atop the cylinder head, which are are geared to operate at one-half of the crankshaft speed.  One cam opens the inlet valve and another cam opens the exhaust valve.  This arrangement is quite efficient, and very emissions-friendly.

I wanted to make a few posts to discuss the evolution of the four-stroke engine, particularly with regard to air flow and valve arrangements.  I also wanted to talk about some interesting designs, and some of the many, many ways engineers have come up with over the years get power from an internal combustion engine.  There are an impressive number of variations!  In another post I will discuss engine layouts - straight, Vee, radial, opposed, and some real oddballs like U, W and X engines..

Before we get going on that however, there are some other engine basics that need to be discussed.  One of these basics is engine compression ratio.  This is how much you compress the fuel/air mixture before igniting it.  Higher compression ratios lead to greater engine power output and torque, but there are trade-offs and limits to how high a compression ratio can get.  In a race engine, with special fuel, you might see 12:1 or 13:1.  In a passenger car, 11:1 would be pretty high, and would require premium fuel.  9:1 to 10:1 are typical.  If an engine has a compressor, such as a turbo or supercharger on it, the compression ratio of the engine must be reduced, because the starting pressure will be greater than a normally aspirated (non-turbocharged) engine.  In a turbo engine, the compression ratio will typically be 7:1 or so.

The problem arising with raising engine compression too high is that the heat of compression can cause the fuel/air mixture to spontaneously combust before the piston reaches the top of its travel.  This leads to a "pinging" or "knocking" sound from the engine.  This causes a great deal of stress on the engine because the shock wave from premature combustion hits a piston that is still moving upwards to complete the compression stroke.  Not good.  Modern engines have sensors to reduce this type of event, but older engines were easily damaged by it.

Next, I want to point out that the intake valves are (as far as I know) always larger than exhaust valves on an internal combustion engine.  This seems counter-intuitive, because you would think all that hot, expanded exhaust gas would need a bigger opening and path, right?  Actually no.  On a non-turbocharged engine, the fuel/air mixture only has atmospheric pressure (about 14.7 psi) to push it into the engine.  With such low pressure, you need a bigger intake runner and a larger valve to allow the air to flow in.  On the exhaust side things are different.  The exhaust is at high pressure and is also being forced out by a rising piston.  As a result, it can pass through a smaller opening.  That is the reason why intake valves are larger than exhaust valves.

Early engines were usually "undersquare", meaning that the stroke of the piston was longer than the diameter of the cylinder bore.  "Oversquare" (short stroke) engines typically develop more horsepower, while undersquare (long stroke) engines typically develop more torque.  Torque is twisting force, while horsepower is a measure of how much work can be performed in a unit of time.

Short-stroke engines can be run at higher speeds, because for a given engine displacement, there is less stress on the rotating parts.  The connecting rods are under great tensile stress as the pistons top out and reverse direction, while the crankshaft has to deal with more leverage in a long-stroke engine. However higher engine speed is directly related to horsepower output, so that is the direction automotive engines tend to go - toward short stroke and high RPM.  Tractor engines tend to be the reverse of that, because they are lugged around just off idle speed.

Long-stroke (undersquare) engines have a longer throw on the crankshaft, and so they "have a longer handle" to turn.  Thus a long-stroke engine makes more torque, or twisting force.  Torque is more pronounced at low engine speeds on any engine.  Depending on engine design, an engine might have more low RPM torque or more high RPM horsepower.

Hit and Miss engines:
Back in the early days of internal combustion engines, before high power output and emissions were important, it wasn't even necessary to mechanically push the intake valve open.  With a weak enough spring, the suction created by the piston as it descended could pull the intake valve open.  This arrangement was common on the "hit and miss" engines that powered farm equipment.  These things remind me of early simple steam engines, except they are powered by expanding clouds of gas instead of expanding steam.

As one might guess from the name, Hit and Miss engines don't have a consistent power stroke.  They might rotate a dozen times before another power stroke "hits".  Hit and miss engines are not equipped with a true speed governor - instead they have a massive flywheel to store momentum until the engine speed drops low, and then a simple mechanism allows the engine to fire.  When the speed is above set speed, a mechanical linkage holds the exhaust valve open, and so the piston cannot create suction to pull in a fresh fuel/air mixture to ignite.  The sparkplug fires each time, but without a combustible mixture drawn into the cylinder, there is no power stroke.  Eventually speed falls off, and the exhaust valve is allowed to close, causing the engine to suck in a single fuel charge, then compress and ignite the mixture.

These engines were dangerous, because the crankshaft and flywheel were exposed, and could certainly damage or amputate a limb.  They were messy for the same reason.  The exposed crank would fling lubricating oil all over the place, where it would accumulate grime.  Modern engines recirculate oil through a pump and filter instead of using a total-loss oil system that slings oil everywhere.

The cooling system on these engines was just as primitive as the lubricating system.  Typically it would be a water bath around the cylinder that would eventually warm up and steam off.  As one would expect, overheating was an issue if the owner was not constantly ensuring the water bath was refilled.  Antique engines of this type often have cracked water bath sides, because the engine was left out in the cold overnight after use and the water froze, expanding and breaking the sides.

In the video below, you can clearly see the very weak spring on the intake valve (right),  You can also watch the connecting rod that prevents the exhaust valve from closing most of the time.  Obviously if the engine were under heavy load, the engine would tend to slow down, the exhaust valve would be closed much more often, and there would be more power strokes (or "hits").

This is a very clever and easy to adjust arrangement.  Just the thing needed for a small industrial engine running in a narrow speed range out on a farm.  It's easy to identify a "hit and miss" engine.  The intake valve won't have anything connected to it but a light spring, and the exhaust valve will have a heavier spring, opened by a rocker and push-rod which will be activated by spinning flyweights.

On an internal combustion engine it is possible to plot horsepower vs. torque, and it is normal for torque to fall off significantly as engine speed increases.  Horsepower tends to increase until engine RPM limits are reached.

In the next post we will look at a few automotive engines.

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