Thinking of Fuel Consumption When Buying a Tractor

Thinking of Fuel Consumption When Buying a Tractor

Instead, the brochures tend to skirt round the subject with soothing words, reassuring us of an engine’s efficiency rather than giving actual numbers.

No single use

This is, to some degree, understandable. Tractors are designed to perform a multitude of tasks in an infinite number of situations, so comparisons would be difficult and, to a large extent, unfair.

This is quite unlike cars which have a single task of getting people from A to B and so comparing fuel consumption figures is a much more valid exercise.

However, all tractors have diesel engines and these will vary in their efficiency according to their design and build, and here there is scope for noting how much fuel is used to produce a unit of power.

As it stands that should be simple enough, and the figures are out there, but there is such a variety of makes and models that to do a full and fair comparison would fill many days, and even more pages.

Does size matter?

There is though, the age old question of the relationship between engine size and fuel efficiency.

Large engines are often considered to provide a better specific fuel consumption than smaller units, if only because of the basic thermodynamic principles.

Yet even this simple belief is now clouded by the further question as to what is meant by a large engine?

No longer about cubic inches

At one time it was simply a measure of cubic capacity, but now we have a given engine block providing a variety of power outputs that a rule of thumb guide is redundant.

Engines are becoming more power dense – that is, more power is generated from smaller displacement blocks – and so we need to consider engine ‘size’ on this figure alone.

As noted, this may not be as simple a task as it appears, yet we can gain some insight by looking at independently published figures that are publicly available, the Nebraska testing station being one such source.

Not all manufacturers test their products here though, so in effect, we are limited to John Deere who do take a variety of their machines along, giving us a chance to see how engine size may effect fuel efficiency within that particular marque.

Revealing results

The table below shows results from tests conducted over the last two years at the Nebraska Test Station

Four models have been selected, roughly corresponding to 100, 200, 300 and 400 horse power, with the maximum power recorded along with the specific fuel consumption.

Specific fuel consumption (SFC), in this case, is the quantity of diesel (kg) consumed per KW hour.

Also included in the table are the figures for the engines running at approximately 60% of full power for comparison.

Efficiency increases with engine size

What becomes quite clear is that the specific fuel consumption does indeed drop as the power rating of the engine increases, and this is at both power levels.

It is also noticeable that all four engines were at their most efficient when delivering their maximum power, and that the difference in SFC between maximum and mid-power decreased as the engines got bigger.

This would suggest that the smaller tractors, which are more likely to be used at lower power levels, lose out on economy by doing so.

Another interesting point to emerge is that the two larger tractors share the same 8.9L engine block and, at full power, they share the same SFC, although a gap opens up in favour of the 8R 410 at 60% power.

Big is best?

The clear message from these results is that the bigger the engine the more fuel efficient it is. However, that only applies when it is working hard.

Although not shown on the table, Nebraska conducted tests at two further power levels, including 20% of maximum. At this level, the 8R 410 produced 6hp less than the 5090E at full bore, but the SFC had shot all the way up to 0.351kg/kW.hr.

The other consideration is total fuel consumption and here the largest tractor used 23.5L/hr to produce 75hp while the smallest took 18.3L/hr to provide 81hp.

Fuel consumption and the buying decision

The consideration of fuel use is an important aspect of buying a tractor. These results suggest that the closer to its maximum potential an engine works, the more fuel efficient it is.