Module 4:

Materials

4.4 How much is needed?

4.4.2 Fuel for machinery

If a compact tractor typically uses the equivalent of 15% of its horsepower (hp) rating in litres of diesel per hour, how many litres of diesel will be used by this 40hp tractor each hour?

1. Do you need some initial help with the calculations?

Let us just understand the first part of the question.

A tractor uses 15% of its horsepower.

If the tractor had an engine with 10 horsepower, then 15% of this is 1.5hp.

10% can be represented as 0.1, whilst 15% can be represented as 0.15. So 10hp x 0.15 = 1.5hp.

Hopefully this can help you now determine how much 15% is of the stated 40hp tractor.

2. Do you need some more help with the calculations?

We need to work out what 15% is of the stated 40hp tractor.

We've shown that 15% can be represented as 0.15. So, we now need to multiply this by the 40hp of the tractor.

10% of 40hp is 4hp (0.1 x 40), so we know it is going to be higher than this value.

So, 15 percent of the figure is: 0.15 x 40hp = 6hp.

3. Do you need some other help with your final decision?

We have shown that 15% is of the stated 40hp tractor is 6hp.

The question is saying that this also means that this value of 6 also relates to the amount of diesel the tractor uses in litres per hour.

There can be several ways to arrive at a correct answer, but hopefully the above approach has been clear

1 This will depend on the efficiency of the engine or power unit.

A tractor might typically consume between 10% and 20% of its horsepower in litres of diesel per hour. So, for example, a 35-hp tractor might therefore consume from 3.5 to 7.0 litres of diesel depending on the engine settings (revs) and the type of work being carried out.

If a tractor, for example, was used for some 300 hours over the year, then a guide estimate for diesel requirement might be 1,500 litres or so.

2 For pedestrian operated equipment a petrol engine might be present, being typically around 4.5 hp, or so. This might consume 1-litre of petrol per hour of activity, so an initial figure of maybe 50-litres might be used.

For equipment with battery power, the electricity use to recharge the battery would be measured in kilo-watt hours (kWh). A battery will usually state the voltage (V), Amps (Ah = Amp hours) and Watts. If the latter is not shown, then this can be calculated by the simple formula of V x A = W. For example, a 56v battery with 7.5Ah would equate to 420 Watts (or 0.42 kW). If the battery was used / recharged for 300 hours over the season, then this would equate to some 300 hrs x 0.42 kW = 126 kWh.

Module progress: 70%

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