Power is important as it determines a tractor’s ability to get through work. However, today, it is increasingly difficult to compare the power output of tractors.

Tractor manufacturers’ brochures are a potential minefield, with power outputs quoted to different measurement standards and at different engine speeds. Sometimes, up to seven different power values are quoted for the one model. This article attempts to offer some guidance through this power puzzle.

Specification

It should be easy; a close read of a tractor specification to find a power value before comparing it with other models. But it’s not that simple.

It can be difficult to compare tractors using the manufacturers’ figures. There are a number of reasons for this.

Engine power boost facilities can certainly confuse the issue. Remember, too, that manufacturers may not want to make it easy for the buyer to compare easily. This is true of many other products and services; think of mobile phone payment plans and health insurance.

For tractors, an understanding of the power figures will certainly help, although it is not the full solution. Manufacturers should be forced to present their power output figures as standard, so that the purchaser knows exactly what a tractor’s power output is.

Why is power important?

Power is the ability to do work and is the most important criteria for categorising and comparing tractors. Typically, a tractor buyer will determine the power category of the tractor to buy and then use the actual power of the tractor to guide the purchase decision.

Of course, there are many other factors in a tractor’s specification that are important, such as weight category, transmission type, hydraulics/lift capacity, cab and controls, but power is the starting point.

There are four contributing factors to the confusion about tractor power figures:

  • Different power rating measurement standards used (DIN, ECE, ISO, etc).
  • The quoting of both ‘‘maximum’’ power and ‘‘rated’’ power in literature and the use of these in model names.
  • The increased availability of a power boost (engine management) facility on tractors.
  • The reluctance of manufacturers to quote/promote the most complete standard tractor test: the OECD test.
  • The glory days

    Today, the ability to compare power is much poorer than it was in the past.

    The best period for tractor comparisons was from the early 1980s to the late 1990s when the DIN 70020 engine power rating standard was almost universally used in manufacturers’ sales literature. Power at rated speed (typically 2000rpm to 2400rpm) was also universally quoted, even though some high-torque engines produced more power when loaded to lower speeds. Power boost facilities had not made any great impression either at this time.

    Comparisons of power from the sales brochure were relatively easy. If you had access to an OECD tractor test report, then PTO power, fuel consumption and torque back-up were easily compared. Since then, however, things have become less clear.

    Power rating standards

    Power is measured on a dynamometer where the engine is loaded (or braked) and the engine’s reaction to that load is measured as torque and power. Unfortunately, there are different test standards which can give different power values for the same engine.

    The ancillary equipment which is connected (eg cooling and electrics) can vary, as can corrections for atmospheric conditions.

    Cooling system load can vary considerably, with some standards not having the fan connected, for example, so different power outputs can be achieved depending on the test standard. Examples of engine power test standards are:

  • ISO TR 14396
  • ECE R24
  • 97/68 EC
  • ECE R 120
  • DIN 70020
  • 2000/25/EC
  • Unfortunately, there are no simple conversions that can be applied to allow tractors tested to different standards to be compared.

    Individual engine designs may react differently to the test standards. However, general trends are clear: an engine tested to ISO 14396 will generally show a higher power figure than one tested to ECE R24, for example. In other words, a tractor rated at 100kW (136hp) from an ISO 14396 is, in reality, less powerful than a tractor rated at a similar power at ECE R24 standard.

    The British-based AEA produced an example of different power outputs when an engine is tested to a number of different standards (Figure 1). But these specific differences are only accurate for one specific engine.

    In this case, clearly the older and now rarely-used DIN standard gives the lowest power output figure, but probably the most realistic for an engine installed in a tractor. So, if you feel that your modern ISO-rated 100kW (136hp) tractor feels less powerful than an older DIN rated 100kW model, you are probably right.

    For the tractor buyer, this situation is ridiculous and unfair. Put simply, manufacturers/trade bodies at EU or international level should agree a standard to be used, or regulatory bodies should force it.

    In the OECD tractor testing code, which unfortunately is not compulsory, engine performance is measured through the PTO, with all ancillaries fitted to the tractor (hydraulics, electrical, cooling systems, etc). This is the best measure of engine performance as it is tested as installed with a wealth of other information produced also, including torque characteristics and fuel consumption.

    Rated power and maximum power

    The rated power of an engine is the power produced at the rated speed of the engine. This is normally where the engine is loaded sufficiently to get the engine into its maximum power range.

    For most modern tractor diesels, this rated speed is between 2000rpm and 2300rpm. In the past, the rated power was frequently the engine’s maximum power. Today’s engines are designed to resist dying under load to such an extent that their maximum power is frequently produced at a much lower speed (1600rpm to 2000rpm) than the rated power speed. This is a good characteristic that accompanies excellent torque back-up characteristics. Today, many manufacturers are choosing to put the emphasis on the maximum power output of the engines with this figure frequently included in the model name. There is nothing wrong with quoting the maximum power figure, but it should only be done to indicate the torque characteristics of the engine and should always be listed with the rated power figure of the engine.

    If you consider the maximum power of a tractor as its usable or rated power, you will be disappointed as it is only accessible at a lower speed, and if the engine is loaded beyond that point, it will die quickly.

    Torque back-up

    Torque back-up is a good indicator of how well an engine holds on to its speed when it is put under load. Good torque characteristics are essential in agricultural applications where load constantly varies.

    It is calculated by dividing the engine’s maximum torque by the torque at the rated speed. Any value above 25% would be classified as good.

    Taking a New Holland T6.120, for example, its brochure rated power is 81kW (110hp).

    The calculated torque back-up from its independent OECD test, using the tractor’s rated power speed, is 42%. This is an excellent figure and is underpinned by the fact that it produces about 8kW or (10hp) more at 1800rpm than its rated 2100rpm.

    But if we want to call that tractor an 89kW (120hp) model based on its maximum power, we calculate its torque back-up based on this power output. We only get a value of 10%, which is very poor.

    So, manufacturers cannot have their cake and eat it. In this example, if they choose to call it a 81kW (110hp) tractor, then it has excellent power delivery characteristics with strong torque back-up. But if they choose to pass it off as an 89kW (120hp) model, then it has poor torque back-up; basically, the power is delivered in the wrong part of the speed range.

    Engine management

    As if it was not confusing enough, a second ‘‘boosted’’ power figure frequently appears on sales literature because, with electronic control, it’s easy to build in a power boost facility. While the engine may cope with the extra power delivery, the rest of the tractor may not be designed for the higher power output.

    Manufacturers can protect the transmission by only allowing the power boost facility in situations where transmission components are not overloaded. These include times when the PTO is in use, or power is being transmitted at higher road speeds.

    When the power boost is available, it is genuine additional power. It can effectively give you the performance of a more expensive tractor, but only in those situations where the power boost facility is available.

    Sometimes, the torque back-up and specific fuel consumption may not be as good as in the non-boosted mode. While the boosted power is of value, remember it is limited to certain operations.

    Making sense of it all

    Information on tractor power given by manufacturers in their sales material can vary considerably. Even within tractor brands, the information on the brochure can vary for different ranges.

    In Table 1, power values from the manufacturer’s literature for three tractor models in the 80kW to 90kW (110hp to 120hp) range are highlighted, along with the real PTO power values as measured in OECD tractor tests. The OECD tests, although only available for some tractors, are the best indicators of true performance.

    This is because the OECD power tests are carried out with the engine installed in the tractor, complete with hydraulic, cooling and electrical systems working.

    The three tractors shown have different engine specifications. Both the John Deere and New Holland models have engine power boost available in certain situations.

    For the new MF5600 range, however, Massey Ferguson chose to have a standard non-boost option. The John Deere 6115R model is tested to the EC 97/68 standard, while both New Holland and Massey Ferguson models are ISO 14396 standard tested.

    Both John Deere and New Holland quote rated power and maximum power. Curiously, the Massey Ferguson data for the MF5600 range has a maximum power quoted, which is very unusual. This is not used in any other Massey Ferguson range where rated and maximum powers are typically quoted.

    With all three tractors, the model number gives an indication of the power output but in different ways. The 115 in the John Deere 6115 name matches the rated horsepower of the tractor in non-power boost mode.

    The 140 in the New Holland model number is close to the maximum power in power boost mode available form this model. The 13 in the Massey Ferguson model indicates the 130hp maximum power.

    So, how do their literature values compare with the OECD tests? All show a much bigger drop in PTO power from their rated engine power compared with tractors of the 1990s and early 2000s.

    This is partly due to the engine test standards being less representative of installed engines than the older DIN standard.

    It is also due to the possible greater power losses through complex transmission and hydraulic systems.

    Overall, the John Deere and New Holland tractor models have similar performances under test although there are some variations. Both have excellent torque back-ups in non-boosted mode, while the John Deere loses slightly on torque characteristics in full power boost mode.

    The Massey Ferguson has a good solid performance for a non-power boost tractor, but the lack of a rated power figure in its specification literature is surprising. While the 5613 gives a good OECD standard pto power output, for a non-boosted tractor, it would be better if the tractor buyer was aware of the rated power of the engine.

    Overall, these three tractors perform well and are reasonably matched. But interpreting that from the sales literature is confusing. The OECD test is necessary in this case to allow comparisons.

    Finally

    The current situation with manufacturers’ information is unhelpful, making tractor comparisons difficult. By far the most useful information on power available to tractor buyers is the OECD tractor test, which gives excellent data on engine/pto power, torque characteristics, fuel consumption, hydraulic and lift performance and drawbar power. These are the figures which the manufacturer should quote. Put simply, the test should be compulsory, as we can see how information has disimproved where manufacturers are allowed to choose their own standards and test criteria.