The Effect of Engineering Innovation on Society

Lawnmowers

Lawnmowers

• Early lawnmowers were heavy and cumbersome, which meant that whilst they were effective, they were still tiring.

• As lawnmowers developed, they became more user-centred. This means that the way they operated did not impact negatively on the user.– Examples such as?

The internal combustion engine

• The cause of most environmental effects associated with the lawnmower (and other powered equipment).

• Lawnmowers use either an unleaded 2-stroke or 4-stroke engine, whilst some commercial models have a diesel power plant.

4-stroke engine

• Why is it called a 4-stroke?

Induction

• In this stage the crankshaft rotates and draws the piston down sucking in and air/fuel mixture through the open inlet valve.

• At the end of the stroke the camshaft will close the inlet valve.

• The air and fuel mixture is mixed by a carburettor on a mower.

Compression

• On the compression stroke both valves are closed and the piston travels back up the cylinder compressing the fuel/air mixture.

• Just prior to reaching the top of its travel the spark plug ignites the compressed mixture.

Power

• This ignition drives the piston back down the cylinder which is where the power is produced.

Exhaust

• Once the piston reaches the bottom of its power stroke the exhaust valve opens and the piston then travels back up the cylinder pushing the exhaust gases out.

• The cycle then repeats itself.

2-stroke engine

• Why is it called a 2-stroke?

The two strokes

• The 2-stroke engine compresses the four strokes into two so that every down stroke of the piston is a power stroke.

• Very effective in small motors, not so in larger motors.

• Tend to be less clean and environmentally friendly because of its oil/fuel mix.

Effect on the environment

• ICE’s operate at an efficiency rate of about 15%, meaning that 85% of the potential energy of the fuel is not used (burned).

• Gaseous bi-products include carbon dioxide and carbon monoxide – both of which are particularly dangerous to humans.

Diesel engines

• Removes the spark plug and ignition system and relies on the heat of the compressed air to ignite the diesel.

• Traditionally more simpler than a petrol engine, modern advances in fuel injection and computerised fuel management mean that simplicity is now gone.

How does a diesel work?

• Still 4-strokes…which were?

How does a diesel work?

• On the induction stroke only air is drawn in by the descending piston. The inlet valve closes and the piston now rises and compresses the air to a higher degree than a petrol motor.

• This compression causes the air to heat up and when the diesel is injected into the cylinder ignition occurs.

• This causes the power stroke, and when the piston travels back up the exhaust valve opens and the gases are pushed from the cylinder and the cycle starts again.

Diesel in context

• Traditionally found use in industrial applications.

• More efficient than petrol engines, with efficiency ratings between 20-25%.

• Equals greater fuel economy and better torque ratings.

The Internal Combustion Engine

• 4 stroke• 2 stroke

• Diesel Engine

Activity

• Choose 3 similar sized cars, one petrol engine, one diesel engine and if possible a hybrid. Compare their emission, fuel economy and performance. Assess the best choice for a range of uses, e.g. long distance touring, city commuting etc.