Big Bang theory

Don't be mistaken into thinking that diesel is the dinosaur of the engine world - it's only just coming of age. This article dispels some myths about how they work, and explains why the modern diesel engine has such an exciting future.

The choice is yours. You can have any type of engine in your construction equipment that you want… so long as it's diesel. With a few exceptions down the decades, this has always been the case - but why is this? Isn't diesel old fashioned, dirty, smoky, noisy and on the verge of being replaced by super-modern hydrogen fuel cells? Well, frankly… no. Modern diesel engines are none of the above; fuel cell technology is only in its infancy and it will be years before they are a regular sight on construction sites - let alone threatening the dominance of diesels. The fact is that recent developments in diesel engine technology have revolutionized almost every aspect of their performance - fuel consumption, emissions, longevity, reliability and power. Far from waiting for retirement, diesel engines have a brilliantly bright future.

The basics

Like petroleum-powered engines, diesel engines are 'four stroke' combustion engines. But unlike petrol, which is a 'spark ignition' that uses an electrical spark to ignite the fuel; diesel uses a 'compression ignition', meaning the increasing cylinder pressure and corresponding heat that are created ignite the fuel. This in turn means that diesels don't need the spark plugs, coils or plug leads that petrol engines use. In fact, a basic diesel engine doesn't need any electricity at all to run. The engine compression ratio is also very high; air is compressed down to about 1/18th of its original size. This is much higher than in petrol engines and means that diesel engines have to be stronger to cope with the higher pressure.

For a given engine displacement (i.e. 8 litre, 10 litre, etc.), diesel's compression ignition system allows more air to be used in the combustion process compared with a similarly-sized petrol engine.  This greater amount of air allows more fuel to be used and results in more power generated for a given engine displacement, compared to a petrol engine. Recent fuel injection system advances now allow electronically controlled direct fuel injection.  These fuel systems replace the traditional mechanical system and result in more precise control of the fuel injection process.  Advantages of these new systems include increased power, lower noise, improved fuel consumption and improved engine response. Also, because these systems ensure that the fuel is burnt very efficiently, the engines emit fewer emissions, thereby reducing their environmental impact.

Power & torque

Whenever you talk about engines you invariable hear the terms Power and Torque. But not many people know more than high numbers are generally better than low ones on machines' specification sheets. So here is a brief recap of the differences:

Torque is the twisting force generated when turning, for instance, a wrench.  English units of torque are foot-pounds (ft-lbs) or inch-pounds (in-lbs).  The international unit of torque is the Newton-meter (Nm). Torque is also considered a unit of work by quantifying a certain amount of force for a given distance.

Power is a measurement of how quickly work can be done. So, if a person lifted a 550lb weight one foot high in one second, one horse-power (hp) of energy would be consumed.  Similarly, one watt is equivalent to applying one Newton of force to lift a weight one meter high in one second.

Power is also defined as a unit of speed (m/s) combined with a unit of force (N). So if something were pushed on a level surface with one Newton of force at a constant speed of one m/s, one watt of power would be consumed.

So in construction terms, engine torque determines how much gravel can be lifted by a wheel loader for a given engine speed.  Power determines how fast the gravel can be lifted. Both are important parameters but what is most important is how well both are matched for the equipment's operating speed.   This is why it is better to have equipment companies who develop and match their own engines for their own applications.

More power!

Naturally aspirated (non-turbocharged) engines use only the quantity of air that is drawn into the engine from the piston movement.  The use of a turbocharger (or supercharger) allows more air to be forced into the engine.  Again, diesel's compression ignition system high compression ratio allows for a higher level of turbo charging than is possible with spark ignition engines.  This higher quantity of air allows more fuel to be used - resulting in more power.

The difference between turbo charging and supercharging is how the device is driven.  Both take air from after the air filter and increase its pressure before introducing it to the engine. A turbocharger uses engine exhaust gases to drive its internal compressor and a supercharger uses an engine-driven belt or gear drive to rotate its internal compressor. But because turbos use 'waste' energy, they are more efficient than superchargers, which need to be driven directly by the engine.

To have even more power from a turbocharger, the air is cooled before introducing it into the engine.  As air is compressed, its temperature rises and it becomes less dense.  By cooling air, its density increases, which results in more oxygen becoming available for combustion.  This optimization of combustion air and fuel has resulted in diesel engines having higher fuel consumption efficiency.

Extending durability

Not only are diesel engines powerful and efficient, due to their inherent simplicity and strength, they are also very durable. But that's not to say that their longevity can't be improved upon. There have been developments in both cooling and lubricants, resulting in a reduction in corrosion, erosion and signs of wear. Water may be a ubiquitous resource but on its own it can cause problems within an engine's cooling system. Such defects can include rust, scale, acidity (leading to corrosion) and pitted cylinder liners. This last problem can be caused by cavitation, where tiny bubbles in the water implode (explode inwardly) when the cylinder fires - and these tiny explosions wear away at the lining of the engine. Scale reduces the engine coolant flow, resulting in higher engine operating temperatures and corrosion may weaken engine components. However, keeping cooling systems clean, along with regularly replenishing with conditioners (which form a protective lining) can reduce build-up of scale and engine corrosion. Care needs to be taken with conditioners though, as it is possible to have too high a concentration.

The same is true with engine oils, which reduce friction, cool the engine and remove contaminants from the engine.  Oil needs to be regularly flushed through and replaced with the correct oil grade. Not all oil is of the same type or quality, so it's important to use oils that have been developed for the particular engine's requirements. Using the recommended oil and regularly changing oil and filters are key factors for a long engine life. Another key factor is the air filter, which prevents engine contamination.  This is particularly important in a construction environment, which can have extreme levels of particulates in the air.  Care must be given in the exchange of air filters in a construction environment to prevent system contamination.  Not changing the filter regularly - or not using the recommended replacement - can lead to a reduction of the incoming air, which in turn can result in high fuel consumption.

Entering a new era…

Diesel engines are undergoing rapid and sustained change; often driven by new exhaust emissions legislation which require drastically reduced engine emissions of particulates, nitrous oxides (NOx) and carbon monoxide (CO). Proving that 'necessity is truly the mother of invention', recent engine design innovations include improved turbocharger performance, flexible high-pressure fuel injection and engine control systems.

Engine manufacturers are currently trying to find the best way to meet the stringent emissions regulations while at the same time continuing the efforts to improve performance and efficiency. Volvo's new solution is V-ACT. Standing for Volvo Advanced Combustion Technology, it is being introduced to meet the requirements of the forthcoming Tier 3 (US) and Step 3 (EU) off highway emissions legislation. V-ACT combines reliable technology with a proven base engine.

Volvo is fortunate in that it is among a small number of manufacturers that make both engines and construction equipment. This means engine performance is tailored to the requirements of each piece of Volvo equipment, rather than having to adapt the equipment around the performance characteristics of other suppliers' engines. Engines are not just matched to the drive train, but also to the hydraulics; providing the response characteristics operators prefer for each equipment type. This produces engines that deliver optimized power and torque when and where needed, as well as smooth throttle response, low fuel consumption, low emissions and (of increasing importance) low noise.

The King is dead, long live the King

Far from being a relic of the past, the diesel engine is now truly coming of age - and is set to cause a few surprises. New combustion technology, low emissions and fuel consumption coupled with high productivity, responsiveness and reliability mean that diesel has become the most efficient of current engine technologies. The King is Dead, Long Live the King!