Adventist Youth Honors Answer Book/Vocational/Small Engines

Small Engines
General Conference
Skill Level 2
Year of Introduction: 1975

1. Describe the design and operation of the two-cycle engine and the four-cycle engine.


The four-cycle engine is easier to understand, so it is presented here first. Once the four-cycle engine is understood, it is easier to understand the two-cycle engine.

Four-cycle Engine

Four-cycle engine

A four-cycle engine is so named because it makes four strokes to generate power. These strokes are as follows:

  1. Intake Stroke: During this stroke the piston moves down in the cylinder, which creates vacuum, drawing air through the carburetor, and sending the mixture of fuel and air into the cylinder. The fuel and air enters through the intake valve which opens at the beginning of this stroke.
  2. Compression Stroke: Once the fuel and air have been drawn into the cylinder, the intake valve closes and the piston returns towards the engine's head. This compresses the fuel.
  3. Power Stroke: When the piston reaches the top of the cylinder (just before Top Dead Center or TDC), the spark plug fires, igniting the compressed fuel and air mixture and causing an explosion. This explosion causes the gases to expand which powers the piston downwards again. This is where the engine's power comes from.
  4. Exhaust Stroke: When the piston reaches the bottom of the cylinder, the exhaust valve opens, and as the piston returns to the top of the cylinder again, the exhaust gases are pushed out. When the piston reaches the top again, the four cycles are ready to repeat.

Two-cycle Engine

Two-cycle engine

Power and Exhaust Stroke: In a two-cycle engine, the spark plug fires every time the piston reaches the top of the cylinder. This is the beginning of the power and exhaust cycle. As in the four cycle engine, the spark plug firing causes the compressed air/fuel mixture in the cylinder to explode, and the expanding gases drive the piston downward. As the piston goes down, it compresses the air/fuel mixture beneath the piston inside the crankcase. The crankcase is the area beneath the piston, and the cylinder is the area above the piston. When the piston nears the bottom of this stroke, it uncovers an exhaust outlet (port) and it also uncovers a passage between the crankcase and the cylinder. The compressed air-fuel mixture in the crankcase then rushes through this passage into the cylinder, forcing the exhaust gases out of the cylinder through the exhaust port. Some of the air/fuel mixture is also forced out the exhaust outlet, and this unfortunately decreases the engine's fuel efficiency and increases its pollution output. The piston's head is shaped in such a way as to minimize this waste, but it does not entirely eliminate it.

Intake and Compression Stroke: After the piston reaches the bottom of the stroke, it begins traveling upwards again, compressing the air/fuel mixture that rushed into the cylinder from the crankcase during the previous stroke, and simultaneously drawing more air and fuel into the crankcase. By the time the piston reaches the top of the cylinder, the air/fuel mixture is very highly compressed - much more compressed than it was in the crankcase. Just before the piston reaches the top of the cylinder (TDC), the spark plug fires, beginning the cycle over again. As soon as the air/fuel mixture is ignited, and the piston begins to move down, the pressure change in the crankcase closes the intake valve, which is located in the side of the crankcase, underneath the carburetor. On most 2 cycle engines, a reed valve is used, which is simply a piece of thin spring metal that acts as a one way valve.

Since the crankcase in a 2 cycle engine is used as part of the intake system, it can't be filled with oil like a 4 cycle, so another way to provide oil to the engine has to be used. If adequate oil wasn't provided, the engine would soon get so hot that it would begin tearing itself up, fuse together and destroy itself. This is solved by mixing special oil into the fuel before it is put in the tank. You must use oil labeled for 2 cycle engines, or you will ruin the engine. The normal ratio specified by engine manufacturers is usually between 16/1 and 50/1 (50 parts of fuel to 1 part of oil) depending on the engine. The newer the engine, the higher the ratio (less oil) usually is. On some engines, outboard boat motors especially, Oil Injection is used, where oil from a separate tank is injected into the gasoline just before the carburetor. This results in a cleaner burning engine. Since the oil is burned, this unfortunately increases the pollution levels generated by two-stroke engines as compared to a four-stroke engine.

2. Name the parts of the two-cycle engine and tell what each part does.

  1. Spark Plug: The spark plug produces an electrical spark igniting the air/fuel mixture as the piston reaches the top of the cylinder.
  2. Piston: The piston travels up and down inside the cylinder. Its downward motion is propelled by an explosion of air and fuel ignited by the spark plug.
  3. Rod: The rod connects the piston to the crankshaft.
  4. Crankshaft: The crankshaft converts the piston's up-and-down motion into rotary motion and transfers the engine's power to the outside of the engine.
  5. Cylinder: The cylinder is the space inside the engine block in which the piston moves. Combustion occurs at the top of the cylinder.
  6. Crankcase: The crankcase is the cavity in the engine block beneath the cylinder, and is where the crankshaft is. This is where fuel is drawn into the engine prior to being forced into the cylinder.
  7. Intake Port: The intake port is the passage-way between the crankcase and the cylinder.
  8. Intake Valve: The intake valve opens to allow the air/fuel/oil mixture to enter the crankcase during the piston's up-stroke, and closes to prevent its escape, and cause it to pressurize during the piston's down-stroke.
  9. Exhaust Port: The exhaust port is a hole in the cylinder wall through which the burnt fuel (exhaust) is expelled.
  10. Flywheel: The flywheel is a mass attached to the crankshaft. It preserves the engine's momentum, keeping the crankshaft turning between the explosions in the combustion chamber. It also has a magnet in one side, which passes by the magneto, generating the electricity to fire the spark plug.
  11. Magneto: The magneto sits next to the flywheel, and has a coil that acts like a generator when the magnet in the flywheel passes it. In older engines, the power from the magneto coil is switched by points, to control the timing of the spark. On newer engines, it uses electronics to control the spark, by sensing where the magnet on the flywheel is.
  12. Fuel Filter: The fuel filter captures dirt in the fuel, preventing it from entering the carburetor.
  13. Carburetor: The carburetor is the device that mixes the fuel and air together prior to its being transferred into the engine.

3. List four basic fuels used in small engines, and explain their use.

  1. Gasoline is the most common fuel for small engines, especially those found on lawnmowers, snowblowers, tillers, and other garden equipment.
  2. Gasoline/oil mixture is used in two stroke engines. The ratio of gas to oil is usually in the range of 16:1 to 50:1. Two stroke engines are often found on smaller items such as chain saws and weed trimmers.
  3. Diesel fuel is sometimes used in larger "small" engines, such as those found on riding mowers, small tractors, or electric generators.
  4. Nitromethane is used for very small engines, such as those that power Radio Controlled model airplanes.

4. Show care and safety in fuel handling and storage.


References: Petrol Canada

  1. Store fuel only in government-approved containers.
  2. Do not leave it in a car or in the back of a truck when filling it - place it on the ground. This will prevent the build-up of static electricity which can ignite the fuel.
  3. After fueling an engine, wipe up any spilled fuel, and allow any remains to evaporate before starting the engine.
  4. Always move the engine at least 10 feet from where you filled it, and the gas can before starting it.
  5. Never add fuel to, or open the filler cap on or near a running engine.
  6. Do not smoke or allow anyone to smoke within 50 feet of an open fuel tank.
  7. Do not fill the container more than 90-95% full to allow room for expansion.
  8. Do not store a fuel container in the trunk of a car, in direct sunlight, or near an open flame or source of sparks.
  9. Keep the fuel nozzle in constant contact with the container when filling it. Keep the container's nozzle in constant contact with the fuel tank when adding fuel to the engine.

5. Describe three types of ignition systems.


Material for this section was adapted from the Wikipedia article on Ignition Systems

Glow plug ignition


Glow plug ignition is used on some kinds of simple engines, such as those commonly used for model aircraft. A glow plug is a coil of wire (made from nichrome normally) that will glow red hot when an electric current is passed through it. This ignites the fuel on contact, once the temperature of the fuel is already raised due to compression. The coil is electrically activated for engine starting, but once running, the coil will retain sufficient residual heat on each stroke due to the heat generated on the previous stroke. Glow plugs are also used to aid starting of diesel engines.

Magneto system


The simplest form of spark ignition is that using a magneto. The engine spins a magnet (normally cast into the outer edge of the flywheel) past a coil, and also operates a contact breaker called "points", interrupting the current and causing the voltage to be increased sufficiently to jump a small gap. The spark plugs are connected directly to the magneto coil. Magnetos are not used in modern cars, but because they generate their own electricity they are often found on small engines such as mopeds, lawnmowers, snowblowers, chainsaws, etc. where there is no battery, and also in aircraft piston engines.

Battery Ignition


Battery ignition systems are very similar to the magneto ignition system, except that electricity is drawn from a battery rather than being induced by rotating a magnet past a coil. The battery is recharged by the engine's alternator. Battery systems are capable of delivering sparks to multiple cylinders which are selected by a rotor in the distributor. The advantage of a battery ignition system is that the timing of the spark may be adjusted to increase the engine's efficiency based on how fast it is turning. Older battery ignition systems used breaker points (often referred to as the "Points") which were opened and closed by a cam run off of the crankshaft to control the timing of the spark. The shortcoming of this system was that the timing stayed the same for all engine speeds, but the engine needs different timing for high speed than at idle. To make up for this, various engines used various systems to adjust the timing, such as vacuum and centrifugal advance, which always had problems. Now almost all engines use Electronic Ignition, which has a sensor on the engine that tells the electronics where the engine is, and how fast it is going, and adjusts the timing accordingly.

6. Explain why gasoline is an improper cleaning fluid.


Gasoline is extremely flammable and dangerous to work with. It is also a carcinogen (cancer-causing agent), so prolonged contact with it should be avoided. It can also damage plastic parts. Gasoline left on parts can burst into flame at the slightest provocation, causing an engine fire. It also has many additives, some of which stay on the part after you clean it. They also soak into your bloodstream.

7. List two acceptable cleaning fluids for small engines.


Mineral Spirits are often used for cleaning parts. Soaking parts in mineral spriits removes oil and grease without the need for scrubbing. Once the parts are removed from the bath, the mineral spirits quickly evaporate.

Environmentally Friendly Alternatives: There are many degreasers on the market today, and many of them are billed as "environmentally friendly." Examples include Simple Green, and various citrus-based cleaners containing limonene such as Citrus King, and Orange Power. These cleaners do require scrubbing, but they are all biodegradable.

8. List and tell how three basic lubrication systems operate.

  • Splash: In a splash lubrication system, dippers on the crankshaft are repeatedly submerged and lifted out of the oil in the oil pan. This happens as the crankshaft turns. When the dippers emerge from the oil bath, oil is splashed all about the inside of the chamber where it finds its way to the pistons, rods, and other moving parts. Channels are milled in the crankshaft to direct oil flow from the dippers to the bearings in the crankshaft, as it is vital that these receive plenty of lubrication.
  • Pressurized: In a Pressurized lubrication system, an oil pump sucks oil from the oil pan and forces it through a filter and then on through a system of passageways drilled into various parts of the engine. These passageways are called galleries. The galleries open to spurt the oil onto critical components such as the bearings. Galleries are also drilled from the main block and from there into the crankshaft's main bearing. This connects to another gallery drilled into the crankshaft itself. The crankshaft gallery supplies oil to the connecting rod bearings.
  • Combination Splash/Pressure: As the name suggests, a Combination Splash/Pressure lubrication system is a combination of the two systems described above. The connection between the rod and the piston, as well as the cylinder walls, are lubricated using the splash method. The main crankshaft bearings, as well as the camshaft, rocker arms, Etc. are lubricated by the pressurized system.
  • Dry Sump: In a Dry Sump lubrication system, there is no oil in the oil pan. It is all stored in a pressurized reservoir, and then fed to the engine by the pump. Galleries are drilled in the crankshaft just as in the Pressurized system. But these galleries are also connected to galleries drilled in the connecting rod, and then through the pistons themselves. Oil is squeezed through these galleries where it spills out onto the cylinder wall.

Reference: Integrated Publishing

9. List in order the steps of a general trouble-shooting procedure.


Remember the acronym FAST - Fuel, Air, Spark, and Timing.

  • Fuel: Make sure fuel is getting into the combustion chamber. The most obvious thing is to make sure the engine is not out of gas. Then check the fuel lines, making sure they are in good shape and actually connected to what they should be connected to. If the engine is flooded (that is, it has too much fuel in the carburetor), let it sit for a while, and then try starting it again. If the engine has a fuel pump, make sure it's operating. Check that the filter is not clogged. A very common problem with small engines, especially after they have been sitting for a while, is for the carburetor to get clogged, or have the diaphragms get old and stiff. A normal carb is not very difficult to dissemble and clean, and rebuild kits can be obtained for a few dollars and will often solve the problem if you are still having problems after you check everything else. If all of these check out, move on to the next stage in the acronym, which is air.
  • Air: Make sure the choke is adjusted properly. On an engine with a manual choke, close the choke all the way and open up the throttle. Then try to start it. If it doesn't start after a couple of pulls, open the choke halfway and try again. When the engine starts, open the choke over the next few seconds until the engine runs smoothly. If it still won't start, check that the air filter is not clogged.
  • Spark: Check that you have spark, and that it is a good strong blue spark. You can use a spark tester, unscrew the sparkplug and hold the outer threaded part against the metal of the engine as you crank the engine and watch for a spark, or just hold the end of the spark plug wire 1/8" to 1/4" inch away from the metal of the engine as you crank the engine. Be careful not to touch the metal tip! The shock will really hurt, and it could cause serious injury, especially if you have a pacemaker. If it is an older style engine with points, you should check them. After a while, they can get dirty or corroded, or just wear away. They are often under the flywheel. Electronic ignition engines can have the module burn out. Or you could have a loose wire somewhere, or the kill switch could be shorted.
  • Timing: On small engines, you normally can't adjust the timing, but that doesn't mean it can't be wrong! It is common on certain engines (Briggs and Stratton for instance) for the key holding the flywheel to the shaft to shear, (they are designed to do this to protect other parts of the engine, kind of like a fuse), and when it shears, the flywheel turns on the shaft. so you will still get a spark, but it won't be at the right time. so you should check the flywheel is in the correct position on the crankshaft. Engines that use a point style ignition system sometimes have an adjustment on the points, which are usually under the flywheel.

10. Demonstrate that you can overhaul, inspect, and properly tune any small engine.


Consult with a Qualified Person for this part! Don't just tear into the family lawnmower, and hope you can get it all back together. You are likely to end up with "extra" parts left over when you are done! (which is not a good thing)

11. Demonstrate that you know how to test and clean spark plugs and glow plugs.


A toothbrush can be a good tool to clean spark plugs. (Don't use it to brush your teeth afterwards though!) You also need to use a spark plug gap gage, or a feeler gage to set the spark plug gap according to the engine specs. When cleaning them, you just need to get all the carbon and soot buildup off. These days, sparkplugs are usually just replaced if they are full of crud.