A century of progress in diesel engine technology
Friday, May 1, 2015
Diesel engines have come a long way since the earliest model designed by Dr. Rudolf Diesel. Fuel consumption of Tier 4 tractors is significantly lower and today's heavy-duty diesel-powered trucks are 98 per cent cleaner than even those of 10 years ago
by RALPH WINFIELD
In the early 1900s, very crude compression-ignition (diesel) engines were developed to burn low-quality fuel. The earliest engine designed by Dr. Rudolf Diesel was a single-cylinder, two-cycle engine which would burn low-grade kerosene that was much cheaper than gasoline.
A few of those single-cylinder engines were exported to North America, but none were ever built here. A number of North American companies started building multi-cylinder diesel engines using indirect injection systems. This injection arrangement, where a relatively crude spray was directed into a pre-combustion chamber to start the fuel burn, was used by a number of early engine manufacturers – Buda, Caterpillar, International Harvester and others.
These relatively crude injection systems permitted the use of low-quality diesel fuel, with a low Cetane number, to power slow-speed diesel engines. Variable injection timing to compensate for the time delay of fuel burn start was just not possible. Thus, the maximum diesel engine speeds were limited to the 1,500 r.p.m. range.
In 1996, the Environmental Protection Act (EPA) in the United States required that the sulphur content of diesel fuel be reduced significantly. As you know, when sulphur is oxidized, it becomes sulphur dioxide (SO2). When water or moisture is added the SO2 becomes H2SO4, which is sulphuric acid. When that acid is released into the atmosphere, it is a major cause of acid rain.
Some of you will remember the problem of acid rain on white beans along Ontario's 401 corridor. The beans were bronzed and many did not survive or yield well.
An act to impose cleaner diesel fuel effectively reduced the sulphur content of diesel fuel by 97 per cent. This sulphur content reduction in diesel fuel caused significant lubricity problems in many diesel injector pumps and injection systems. Fortunately, the diesel engine/pump manufacturers rose to the challenge and modified their equipment.
The tier system. The EPA mandated a tier system for the reduction of nitrous oxides (NOx) and particulate matter (PM), primarily carbon from diesel engines. Initially, the tier system was only applied to highway vehicles but, as it progressed, the same tier system was imposed on "off road" vehicles. These vehicles included construction and agricultural equipment powered with diesel engines.
Tier 1 was initiated in 1996 for compliance by 2001. Tier 2 was applied to road-going vehicles from 2001 to 2004. The upgraded Tier 4 requirements, which were staged and also applied to off-road vehicles, were introduced in 2011.
The staged Tier 4 requirements were to be fully implemented in 2014 and they were to be applied to all on- and off-road diesel engines over 175 horsepower. It was recognized that smoke (carbon) and nitrous oxides from off-road engines were equally damaging to the environment.
Most diesel engine manufacturers started to take the EPA requirements very seriously, starting in 2006.
By 2006, virtually all diesel engine manufacturers had switched to direct injection engines. This design change was accomplished by having much finer atomized diesel fuel sprayed directly into each main combustion chamber. Gone were the pre-combustion chambers that accepted much coarser spray cones from the injectors. Those cones were often directed horizontally across the top of the combustion chamber to guide the coarse spray into the pre-combustion chamber on either side of the main combustion chamber, directly over the piston.
The more effectively atomized spray pattern from a number of very small orifices provided much smaller fuel droplets that would be ignited faster by the heat of compression. In other words, the time lag of ignition was reduced significantly.
The Cetane number of the diesel fuel is a rating of the diesel fuel's ability to start the fuel burn. It must be higher for the direct injection engines. Pure Cetane has a rating of 100. This is the direct opposite of the Octane number of gasoline, which is a measure of the non pre-ignition capability of gasoline in a spark ignition engine.
This injection system change allowed for better fuel efficiency and higher speed engines. The engine speeds increased from about 1,500 r.p.m. into the 2,500-3,000 r.p.m. range. This also allowed for the start of injection advance systems that had not been possible with the older indirect injection systems because they had a significant time lag to full fuel burn.
Many of these newer direct injection diesel engines could also be equipped with exhaust-powered turbochargers, which permitted more oxygen to be pressured into the cylinder. This additional oxygen permitted more fuel to be injected and thus more horsepower to be produced from an engine of a given displacement.
The turbocharged air was increased in temperature, so next came intercoolers which reduced the compressed air temperature and effectively improved the "volumetric efficiency" and the output horsepower of the engine. Unfortunately, these power increases did nothing to clean the diesel exhaust gases.
Tier 3 environmental standards. In the May 2006 issue of Better Farming, Keith Berglind wrote an excellent article explaining and illustrating the equipment modifications used to meet the Tier 3 Standards. The key factors were:
- Variable Geometry Turbochargers (VGTs). An engine controller sets the pitch or angle of the turbocharger blades to change the output of the turbocharger.
- Cooled Exhaust Gas Recirculation (EGR). The EGR system takes a portion of the exhaust gas from the turbocharger, cools it by an intercooler and reintroduces it back into the intake air stream. This action assisted in reducing the nitrous oxides (NOx) in the exhaust gas stream.
- Most newer engines had better centred injectors and many had four valves per cylinder. This allowed for much finer atomization of the injected diesel fuel and an earlier start of the fuel burn to reduce carbon emissions.
Basically, these design changes permitted much higher injection pressures and finer spray droplets, which provided better low-speed torque and faster response times.
Injection pressures in perspective. When many of us tested injectors in the 1950s, we would see reasonable cone-shaped spray patterns in the 1,200-1,800 pounds per square inch range. This was very acceptable for the older, slow-speed indirect injection engines.
With the newer common rail injection systems, we are seeing injection pressures approaching the 2,400 bar level. For you non-Europeans, a bar is one atmosphere of pressure or 14.5 pounds per inch squared. A 2,400 bar pressure will be 2,400×14.5 = 34,800 pounds per inch squared! The resultant very fine spray allows the newer electronic control systems to do absolute wonders when coupled with engine speed variation and power response systems.
Tier 4 final compliance, 2014. First, this level of compliance is only required for tractors and other engines of 175 horsepower or higher. However, when you look at all those big tractors travelling the roadways and working in the fields, they do come in all colours. But let us not forget that many combines and other harvesting units are also powered by engines of 175 hp or more!
Today's clean diesel-powered heavy-duty trucks are 98 per cent cleaner than those built 10 years ago. Do you remember when I reported in an earlier article that there was a significant push to build more trucks before the end of 2007 to eliminate the need for all the emission reduction equipment? (See Better Farming April 2011.)
Well, guess what? The Diesel Emissions Reduction Act (DERA) in the United States provided much of the funding for the EPA emissions reduction requirements to meet present Tier 4 standards.
In December 2014, a 50 per cent budget increase for the DERA was proposed to reduce emissions from older diesel trucks still in use.
The reduction in diesel fuel consumption is so significant with the new Tier 4 tractors that most manufacturers are voluntarily applying the technology they have gained to their lower horsepower tractors. The environment and all of us will benefit from the legislation that was imposed. Now that is a real success story that we can share with our children!
By way of a final comment, it is absolutely critical that the Diesel Exhaust Fluid (DEF) not be contaminated by being transferred from large containers using old or used containers such as oil or other fluid jugs. BF
Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.
Understanding the abbreviated terminology
Ultra Low Sulphur Diesel (ULSD) and Exhaust Gas Recirculation (EGR) are both relatively easy to remember.
Selective Catalytic Reduction (SCR) means that a type of catalytic converter is in place and it will require Diesel Exhaust Fluid (DEF) to rejuvenate the catalytic converter. The DEF will be carried in a separate tank on the tractor/combine.
The Diesel Exhaust Fluid (DEF) is used to effectively reduce the nitrous oxides (NOx) used with the SCR technology. DEF is injected into the exhaust chamber, where it converts to ammonia (NH3) and carbon dioxide (CO2). The NOx created by diesel combustion reacts with the ammonia and carbon dioxide in the presence of the SCR catalyst, forming harmless water and nitrogen gas which are released as exhaust gases.
The rate of DEF usage is usually less than 10 per cent of diesel fuel consumption. Thus you will see about a 10:1 ratio of diesel fuel to DEF tank sizes.
Last but not least are the Coated Soot Filters (CSF), which will require replacement after a prolonged period of use.
Continuously Variable Transmissions (CVT) or power-shift transmissions became a necessary feature in permitting the Electronic Control Unit (ECU) to select the best engine speed thereby ensuring that the engine is running at peak efficiency to match the load requirements. BF