Diesel engines have often been tarnished with the reputation that they are the noisy, dirty, and antiquated smoke billowing animals of the engine world. The truth about today’s engine is, however, quite the opposite. Advanced electronic engine management systems, improvements in fuel injection design, air flow and fuel management, along with post combustion treatments have overcome many of these previous design flaws and the resulting stigma that was associated with these engines. In fact, current diesel engine designs produce less harmful emissions per given volume of fuel burned than a gasoline powered engine of similar horsepower. The introduction of these new technologies has, however, presented the industry with a number of additional challenges in order to maintain a reliable engine.
Much has changed in the design of diesel engines, and today’s engines weigh half as much, produce nine times the power and twenty-eight times the fuel injection pressures as they did in the 1930’s. Such enormous gains have not occurred without some pretty remarkable technological advances in engine design along with advancements in fuel injection and the actual science behind the internal combustion process.
The diesel engine has been a vital workhorse for many industries around the world, powering large trucks, farm equipment, recreational vehicles, railroad equipment, marine vessels, construction, and mining fleets. Diesel engine exhaust do, however, contain harmful pollutants in a complex mixture of emission gases and particulates, which are known to be harmful to the environment and humans. These emissions include:
Oxides of Nitrogen (NOx)
-- Gases that form when diesel fuel is burned with excess air.
Diesel Particulate Matter (DPM)
-- Microscopic solid particles and liquids that form during the combustion process.
-- Gaseous compounds that result from unburned fuel and lubricating oil.
Carbon Monoxide (CO)
-- Colorless, odorless gas produced when hydrocarbons in diesel fuel are not burned completely.
Contrary to popular belief, diesel engines only emit a small amount of Hydrocarbon (HC) and Carbon monoxide (CO), so engine manufacturers primarily focus on reducing NOx and DPM. These two types of emissions are inversely related; meaning a reduction in one generally causes an increase in the other. Such a relationship complicates the management of them.
The movement towards reducing diesel emissions began in the 1970s & 80s with the USA EPA investigating engine gaseous emissions from the heavy-duty highway diesel engines. The regulation of harmful exhaust emissions began in 2000 when the levels of NOx and DPM began to be controlled under a “tiered” series of emissions regulations which mandated a maximum level. A huge step forward was taken in 2000 when standards issued by the USA EPA directed very large reductions in exhaust emissions for model year 2007 heavy-duty engines. These tiered structures now govern new static and off-road engines and equipment, and establish progressively lower allowable emissions of NOx and DPM.
Each progression of standard level (Tier 1, Tier 2, Tier 3 and Tier 4) required engines to produce lower emissions, and neccessitated more advanced technologically than the previous generation. Essentially, the standards require that manufacturers reduce the levels of DPM and NOx in four steps, resulting in 96% lower levels than the levels produced by diesel engines that were available in the late 1990s. It is important to note that today’s Tier 4 emission requirements apply to new products only and do not apply retroactively to any existing machines or equipment.
Courtesy of MTU
By far the greatest advancement over the past 10 years in engine design has been the development of the High Pressure Common Rail (HPCR) fuel injector. These remarkable devices have been the cornerstone for advances in combustion efficiency and allow an engine to achieve the stringent new emissions levels proposed by the engine Tier ratings.
While the HPCR fuel injector is a more recent development in engine design, the concept of common rail fuel injection is not. In fact, common rail fuel injection has been around in a basic form since the early 1920’s.
Modern HPCR fuel injection systems are simple in their overall design. However, they comprise a number of highly complex interacting key components in order to function reliably and efficiently. The main HPCR components are listed and illustrated in the image below. Unlike older Electronic Unit Injector (EUI) designs, the HPCR fuel injector does not develop its own pressure during engine operation.
The pressure for injection is developed by the high-pressure fuel pump (1), which is mechanically operated by the engine. A common fuel rail (3) connects each injector from the fuel pump; hence where the terminology of Common Rail originated. The design of this system is such that a constant fuel pressure is available at the injector 100% of the time during engine operation thus providing a higher available mean time injection pressure over EUI designs. This feature ensures that fuel droplets are atomized the moment they leave the injector nozzle. As EUI systems must develop the fuel pressure during the injection event, they have a tendency to form larger droplets at the start and end of each injection event. HPCR injection systems are also able to regulate fuel injection pressure based on engine requirements, speed, and duty using the engine ECU combined with the fuel pump.
The modern HPCR fuel injector is unlike its EUI predecessor, and to understand why they are more susceptible to contamination related problems it is important to appreciate how they function during engine operation. Unlike EUI systems, which typically inject fuel once or twice per engine revolution, HPCR fuel injectors can provide up to 5 injection events per single compression stroke of the engine.
Putting this into perspective, with a large high-speed diesel engine operating at 1400 RPM, the fuel injector is capable of injecting fuel into the combustion chamber in varying quantities depending on the ECU outputs, up to 1,750 times per minute, or 29 times per second with the fuel exiting the injector tip at speeds in excess of 700 miles per hour.
Poor quality or contaminated diesel fuel contributes to more than 80% of fuel system related failures in EUI engines and an even greater amount in HPCR systems. In order to overcome the challenges of operating a reliable fuel system, a paradigm shift must be made in the way consumers view the diesel fuel and its function within the engine.
Historically, consumers of diesel fuel have generally purchased, stored, and distributed the fuel to machines, engines or marine vessels as required with little thought put into contamination control. Over the past 60 years, little has changed in the way many diesel engine owners and operators have undertaken this process. However, with the introduction of these new engines with advanced fuel injection systems, many users are experiencing a high frequency of failures, decreased availability, and increased downtime, as well as cost challenges from a technology that promised to improve operational profitability.
Along with the introduction of HPCR injection systems, we are now starting to see a parallel increase in diesel fuel related problems within many industries including Power Generation, Marine, Agriculture, and Mining. The most common complaint is that the new engine (using HPCR injection systems) simply do not last as long as older engines did (EUI Injection). These issues have created a storm of confusion with fuel suppliers, fuel additive suppliers, filter manufacturers, fuel purification companies, and engine OEM’s, all who seem to have their own opinions or in some cases, magical solutions to offer. Little, however, has been done to educate the end user or provide them with real technical information in relation to the root cause of the problems. Most users are now finding that they are treating the symptoms of these failures rather than actually treating the root cause of them.
Diesel engines are used in a wide variety of applications around the world and with some, they can literally mean life or death or perhaps millions in lost revenue. Such engines are typically found in Mission Critical applications such as data collection or data back-up facilities, hospitals, and the military. With diesel fuel directly contributing to greater than 80% of fuel system failures, it is imperative that these diesel engine users dramatically alter the way in which they view and treat the diesel fuel they consume, from simply seeing it as a commodity or a necessary expense, to that of a "Critical Reliability Component" of the fuel system, the engine, and the overall asset or facility. The cost through poor management of the engine injector system is far too great.