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On Volvo’s new D13A 13-litre engine, the stroke length has been increased for greater cylinder volume and higher torque. The torque has been increased by around 200 Nm and spans a broad speed range. It’s all about torque.

When margins are thin in road transport - as they indeed are - the way a power train is specified and managed means profit or loss. It is thus that FleetWatch technical correspondent, Dave Scott, takes a look at one of the main components of the power train - the diesel engine - to help impart a better understanding of the characteristics of today's modern diesel engine which will assist operators with planning vehicle performance and establishing some of the reasons why operational costs are incurred. Look out in future editions for further 'focused' looks at the other components that make up the power train.

A truck's job is lugging payload and there's no substitute for engine torque when it comes to heavy work. Economy today centres around consistent torque curve management - it's where an engine will use the least fuel expressed in grams/kilowatt hour. Torque tells far more about diesel engine capability than kilowatts or horsepower. An insightful industry observer notes - 'Horsepower is for show, but torque is for GO!'

Why is it that so little is understood about torque among truck salespeople, drivers and their managers? It's probably because truck marketing activities use the language of horsepower - many truck model badges use horsepower as a key characteristic. We like to chat about the '450 horsepower segment' and operators will compare different truck makes using horsepower as the basis.

The 9t GVM breakpoint, where all trucks exceeding this are legally restricted to 80kph, hasn't changed for decades. What has changed is engine torque output that dramatically improves average speed and productivity while fuel consumption has gone down. When ADE engines forced their way onto the market through duty protection in 1981, the ADE 407T was regarded as one of the most powerful options available at 1 200Nm and operated at around 45t GCM. Now - 25 years later - most respectable line-haul engines are operating at 49,5t GCM - a 6-axle artic rig - with engine torque of 2 200Nm. This, of course, has meant that the entire drive train has had to be considerably beefed up to handle so much torque increase.

When it comes to getting a load moved, or a stump pulled out of the earth, massive leverage is required in the effort to gain momentum. That is the moment when the force of torque (Nm), is very distinct from the speed of power (kW). Torque is a measure of leverage - it's a force that tends to rotate or turn things.
 

Maximum torque indicates the most powerful revolution force that an engine generates, which is never at maximum engine revolutions. An engine that generates maximum torque at high r/min always requires the engine to work at high speeds to generate the strongest force - this may work for a rally car but does not apply to economical trucking. Trucks require high torque to come in at low r/min with a wide r/min spread to limit excessive gear changes.

In the case of a normally aspirated (N/A) engine, the specified torque and power of an engine will not remain constant - all specified ratings for N/A engines are measured at sea level for comparison purposes. Turbocharged engines do not experience power and torque loss at altitude, as the turbocharger compensates by increasing speed as exhaust back-pressure decreases with increases in altitude.

With every 100m increase in altitude a N/A engine loses 1% of both power (kW) and torque (Nm). This is not often understood and is one reason why trucks, laden to maximum (GVM), struggle at Gauteng altitudes of 1500 metres where 15% of engine power is lost on a N/A engine.

An alternative approach to understanding power, is that kilowatts or horsepower provide the amount of work done per hour -

Horsepower or kW = Work
                                     Time

The relationship of power and torque is expressed in the formula -

kW = Nm x r/min or Nm = kW x 9554
            9554                          r/min
 
(constant factor) 

Power is measured in kilowatts (kW) and introduces the element of time to the use of torque - it is the measurement of how much work is done in one second. 
         1kW = 1000 Nm per second
The kilowatt output of an engine does not specify force but rather the ability to work, or the potential. The kW output increases as engine revolution speed rises - the maximum kW output is always specified at the r/min speed that this is achieved.

The bore & stroke usually listed under engine specs are relevant to torque. Bore is the cylinder diameter while stroke is the distance a piston moves from the bottom to top of the cylinder. When the bore is greater than the stroke, the engine is regarded as 'over-square' for free revving characteristics. An engine with good torque properties is designed with the stroke exceeding the bore - a 'long-stroke' engine is Hino's successful J-Series engine family that for all 4 and 6-cylinder models, has a bore/stroke of 114mm/130mm. A diesel engine also requires a long-stroke for compression purposes - it is, after all, a compression-ignition engine.
 

Power is a measure of how quickly work can be done. Using a lever, you may be able to generate 150Nm of torque. But could you spin that lever 2 000 times per minute? That is exactly what an engine does.
  

The SI unit for power is the watt (W). A watt breaks down into other units that we have already talked about. One watt is equal to 1 Newton-meter per second (Nm/s). You can multiply the amount of torque in Newton-meters by the rotational speed in order to find the power in watts. Another way to look at power is as a unit of speed (m/s) combined with a unit of force (N). If you were pushing on something with a force of 1 N, and it moved at a speed of 1 m/s, your power output would be 1 watt. 
 

Wyk Cronjé Clover's Manager: Milk Procurement Logistics and Driver Training: "Engine revs equal fuel consumption - driver training is all about understanding the power that engine revs have to make or break trucking economy."

Torque and horsepower are very closely related. A high-revving motor does not need to produce much torque to generate lots of kW. Similarly, a high torque engine may not develop elevated kW levels.

Unseen deadly engine enemy - idling without Nm load
The Professional Truck Driver Institute of America has the following observation in their Tractor/Trailer Driver Handbook: 'Too much idling wastes fuel. When idling, the vehicle is getting zero miles per gallon. Idling can waste as much as 1 gallon of fuel in an hour. It can also clog the fuel injectors. The engine may not be hot enough for complete combustion. The unburned fuel can cause harmful deposits.'

The Handbook points out further that: 'Today's engines do not need to idle more than five minutes. Fleet surveys show that many truckers idle for long periods at truck stops during the warm-up and cool-down periods. One hour of idling causes the same amount of wear as two hours of driving. The typical over-the-road truck idles about 800 hours - this is equal to driving 64 000 miles (102 000km!)."

Leaking air brake systems in heavy diesel powered trucks are a common cause of engine wear. The reason is that drivers rev up cold engines to charge up empty air pressure reservoirs in brake circuits - the rear axle spring brakes will not release until the system is sufficiently pressurised. Drivers and fleet managers must realise that the consequences of leaking air brake circuits extend far beyond loading bays. Drivers will fall into a habit of letting an engine idle all day to keep air brake reservoirs charged - this further aggravates engine wear.

Training is a must
The different torque characteristics of all the modern diesel engines on our market mean that driver training has to be specific to an engine under command. The generic ADE engine days are nearly over. Old engines required increased revs to raise engine torque before releasing the clutch, whereas modern engines provide almost full torque at idle. This simply requires releasing the clutch at idle to move off.

I have watched new trucks leaping around in traffic when the clutch is released because the old rev-'n-release habit is being applied to 'new-torque-technology'. It is management's task to know the torque curve shape of diesel engines within a fleet and then set boundaries of how engine-effort must be managed within a specific torque curve. 

Even better, modern electronically managed diesel engines will allow torque curves to be set for specific routes to achieve optimum fuel consumption and overall productivity. Operators are programming engines to reduce kW while retaining maximum torque and reaping the benefit of reduced fuel consumption.

And then there's the often overlooked data download that these engine provide from their electronic control units (ECU's). If you really want to know how an engine has been treated - or what was happening just prior to an accident - then there's no finer information source than an engine ECU. They are even time and date encrypted now to match incidents to time-frames. It's no wonder that an ECU is sometimes 'stolen' from an engine at an accident site.

Finally, fleet managers must impart all this knowledge to drivers in understandable parameters, allowing them to drive to this formula. The problem is that warnings without training to back up behavioural change fall on deaf ears and have no place in labour legal procedure. Driver training is the only option to make warnings stick. There's no way out! Fleets who want disciplined drivers sticking to critical torque curve parameters must train all staff on this issue. Just stick to the 'sweet-spot' on the correct part of the curve!