The challenge: build the fastest motor racer that is humanly possible.

Car Pros Tried and tested solution. Can take more powerful engine than a trike. Can use parts from various vehicles – less room for mistakes. Cons Heavy. Will have to use whatever wheels can be found to fit.

Trike

Pros

• Super light and a fast mover.
• Simple controls – gives time to make bike go even faster.
• Can use more suitable wheels (thin in front, thick in back) because will be creating from new(ish).

Cons

• Untried and tested – there may be hidden inefficiencies.
• Will have to be built from scratch.
• The engine will be less powerful than the car's.

What is horsepower?
The 18th-century engineer James Watt invented the term 'horsepower'. He was working with ponies lifting coal at a coal mine and needed to find a way to talk about the amount of power these animals could produce.

According to Watt's calculations, one horse is able to do 33,000 'foot-pounds' of work every minute. For example, in one minute a horse exerting one horsepower can raise 330lb (150 kilograms) of coal 100 feet (30 metres), 1,000lb (450kg) of coal 33ft (10m) or 1lb (0.45kg) of coal 33,000ft (10,060m). For greater distances, as in the last example, a block and tackle needs to be used, with an arrangement of pulleys to trade weight for distance.

Horsepower, torque and car engines
A car engine produces torque. When petrol ignites, the mini-explosion pushes on the engine's piston, and the piston exerts pressure on the crankshaft, making it turn. As a result, the crankshaft feels some 'foot-pounds' of torque, or turning force. To move a heavy weight, a lot of torque is required, and acceleration from a standing start is provided by torque.

Torque is affected by the area of the piston face, the amount of pressure that the ignited fuel exerts on it and the distance the piston travels on each stroke. In addition, the larger the crankshaft's diameter is, the bigger the lever arm will be and the more torque there will be.

There is a direct relationship between horsepower (hp) and torque. This can be expressed as:

hp = torque x rpm / 5,252

High values of revolutions per minute (rpm) favour horsepower. Any engine run at very high revs will generate lots of horsepower - even if its torque stays constant. But if two engines produce the same torque, the one that can do it more times per minute does more work and therefore has more power.

 How can car engine horsepower be increased?

• Reduce air resistance as the piston moves down in the intake stroke.
• Improve the exit of exhaust.
• Increase the number of valves per cylinder to improve airflow in and out of the engine.
• Increase the amount of air (and thus fuel) into a cylinder using turbo- and superchargers.
• Cool the incoming air – the cooler it is, the more it will expand when combustion takes place.
• Use lightweight parts to help the engine rev faster.
• Increase the compression rate of the air/fuel mixture – higher compression ratios produce more power.
• Make the cylinders bigger to increase displacement – there's more power because you can burn more petrol during each revolution of the engine.

More power or more torque?
Power is work done – how much is moved how far and how soon. Top speed is determined by power, or the number of torque strokes that can be produced in an engine in the shortest time. If you were to draw up a graph of power output plotted against revolutions per minute (rpm), you would see the power rise with the increasing rpm.

In a trike, less torque is used because there is a lower mass to move. A car, because it's heavier, needs a lot of torque and power, and for drag cars, this is essential. The art of the engine builder is in tuning the engine to adjust the power and torque curves to best suit each application.

What is car transmission?
Cars need transmission due to the design of their engines. Every engine has a maximum rpm value above which the engine can't go without exploding. Also, engines have narrow rpm ranges where horsepower and torque are at their maximum.

Gears are shifted so that the engine can stay near the rpm of its best performance. The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down.

The transmission is connected to the engine through the clutch. The input shaft of the transmission turns at the same rpm as the engine. A five-speed transmission applies one of five differential gear rates to the input shaft to produce a different rpm value at the output shaft.

The best transmission is flexible enough in its ratios so that the engine can always run at its single best-performance rpm value. This is the principle behind the continuously variable transmission (CVT), which has a nearly infinite range of gear ratios.


Find out more from the Scrapheap Challenge adviser about possible solutions for building a drag car and a drag trike.

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