Big Bang Theory
01 May 2008
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, this has always been the case — but why is this? Is diesel not 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 diesel's dominance. Recent developments in diesel engine technology have revolutionised 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.
Like petroleum—powered engines, diesel engines are ‘four stroke’combustion engines. But unlike petrol, which uses an electrical spark to ignite the fuel; diesel uses compression ignition, meaning the increasing cylinder pressure and corresponding heat that are created ignite the fuel. This in turn means that diesels do not need the spark plugs, coils or plug leads that petrol engines use. In fact, a basic diesel engine does not need any electricity at all to run.
The engine compression ratio is also very high; air is compressed down to about 1/18 of its original size. This is much higher than in petrol engines and means that diesels have to be stronger to cope.
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 allows more fuel to be used and results in more power for a given engine displacement when compared to a petrol engine. Recent fuel injection system advances now allow electronically controlled direct fuel injection.
These replace traditional mechanical systems and result in more precise control of the 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, engines emit fewer emissions, thereby reducing their environmental impact.
Power And 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 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 (inlbs). The international unit of torque is the Newton—metre (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 lifts 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 and 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.
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, a diesel's high compression ratio allows for a higher level of turbocharging 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 turbocharging 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. But because turbos use ‘waste’energy, they are more efficient.
To get even more power from a turbocharger, the air can be 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 results in higher fuel consumption efficiency.
Not only are diesels powerful and efficient, due to their inherent simplicity and strength, they are also very durable. But that is not to say that their longevity cannot be improved. 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 engine's lining.
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. 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 is 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 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 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 efforts to improve performance and efficiency. While names vary, the basic principles of almost every manufacturer's current solution remain the same. Optimised combustion efficiency and combustion chamber design, and electronically controlled high pressure fuel injection using common rail technology lie at the heart of many recent new diesels. Exhaust Gas Recirculation (EGR) is another component that will increasingly be seen on diesels as engine manufacturers strive to meet the increasingly tight limits on the emission of nitrous oxides (NOx).
Common rail injection systems inject the fuel into the combustion chamber under high pressure, independent of engine speed or load. Systems normally consist of a high pressure (circa 200-300 Bar) accumulator (the rail), a fuel supply pump, an electronic control unit and a series of injectors connected to the rail. Each injector is opened and closed by the Engine Control Unit (ECU), which also provides computer control to optimise engine performance across the operating envelope, from full speed right down to idle. Injection sequences, including periods before and after the main injection, can be utilised to reduce both emissions, especially NOx, and noise – both principle targets in the forthcoming legislation. EGR reduces the peak combustion temperature by routing exhaust gas back to the intake manifold (it is cooled on route), where it mixes with the incoming air. The has the effect of removing oxygen from the fuel/air mix in the combustion chamber, leading to less NOx being produced by the combustion process.
So how are manufacturers applying the technology? 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 Stage IIIA (EU) off highway emissions legislation. V-ACT combines reliable technology with a proven base engine. V-ACT engines feature new cylinder liners and airto- air intercooling, with some larger units sporting four valves per cylinder and overhead camshafts. Volvo says the keys to the design are the high pressure fuel injectors and the engine management system (EMS) that controls them.
Volvo is among a small number of manufacturers that make both engines and construction equipment. It believes that 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. Volvo does not sell its engines to other OEMs, so you will only ever find V-ACT engines in Volvo construction equipment
Cummins is another that has gone the common rail route with its Quantum range of engines and charge air cooling is another feature that it has applied to some engines in the range. The aim of the company's extensive and long-term R&D effort has been to optimize fuel consumption efficiency. The result of this effort is that many of its existing Tier 2/Stage II compliant engines will require minimum change to met the more demanding future requirements. In common with other manufacturers, much effort has been expended by the company on developing a highly efficient electronic engine management system, a feature it believes is crucial in meeting the legislative requirements.
John Deere's Tier 3/Stage IIIA PowerTech Plus engine range makes use of both common rail high pressure fuel injection and cooled EGR, with a variable geometry turbocharger to help drive the EGR system. As usual, an Engine Control Unit (ECU) is the heart of the engine, controlling the air-to-fuel ratio, multiple fuel injections and the amount of EGR, while at the same time providing diagnostic functions.
Deutz, meanwhile has developed its own version of the Common Rail injection system for its latest generation engines in the form of a twin pump design that will leave the basic engines unchanged. Called the Deutz Common Rail (DCR), this design will be introduced across the full range of Deutz diesels over time.
DaimlerChrysler, owner of both Detroit Diesel and Mercedes Benz, produces off-highway engines for a wide range of applications that meet the Tier 3/Stage IIIa requirements under these brand names. These units feature turbocharging with charge air cooling to reduce the intake air temperature to well below ambient conditions to help reduce NOx emissions. Once again, a high pressure fuel injection system is used, a variation on the common rail theme, which uses a ring fuel circuit with solenoid operated injectors. This, the company claims, provides even lower emissions than standard common rail packages. Naturally, an electronic engine management system is used to control engine performance.
One of the first manufacturers to actually gain Tier 3/Stage IIIA certification is Lister- Petter. Its Alpha LPWS series two, three and four cylinder water cooled direct and indirect injection engines in the power range of 9 to 40 hp (7 to 30 kW) offer both low noise and low emission levels.
Daewoo also is making use of common rail and an optimised engine design in its efforts to reach Tier 3/Stage IIIIA compliance, and CE understands that a three model family is currently under development. Meanwhile, Iveco is also developing Tier 3/Stage IIIA compliant engines using common rail fuel injection and advanced electronic engine management to ensure optimised performance.
Caterpillar has developed its own system, ACERT, which is an advanced package that is built around a simple concept – control the combustion process efficiently and you minimize emissions. Combustion chamber and cylinder head design has been optimized to improve combustion efficiency, and a Mechanically Actuated Electronically Controlled Unit Injection system provides multiple injection and rate shaping capability. This is a patented system that does not make use of the common rail concept. An electronic control module, the ADEM 4, controls all engine operations. Nine ACERT equipped engines are now available following the launch of the new C27 (27 litre) and C32 (32 litre) units designed specifically for off-road applications.
Perkins Engines, which is owned by Caterpillar, also makes use of the ACERT system to ensure its latest generation engines comply with the various regulations. Its most recent launch was models in the 1100 series.
Hatz has also gone a different route for its new 2, 3, and 4 cylinder W35 engine family, which is also available in two turbocharged versions, the 3 and 4 cylinder W35Ts. It covers a power range from 5,2 to 36,5 kW up to a maximum speed of 3600 rpm.
The major difference is the way Hatz achieves high pressure fuel injection. Each individual cylinder is fitted with a fuel pump/valve combination in a single housing, mechanically activated by the camshaft, that injects fuel at a very high pressure (1000 Bar) into the cylinder, thus dispensing with the need for a common rail.
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!