Manufacturers of diesel engines have developed new technology in response to tightened emission standards, continuing the trend toward cleaner-burning, more efficient diesel power for motorhomes.
By Lazelle D. Jones
The milestone achieved in October 2002 underscores how agencies with different agendas can work together to resolve social, economic, and political issues. Those who build diesel engines for motorhomes worked with the federal and state regulatory agencies that determine diesel exhaust emissions standards to achieve that milestone. More specifically, the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB), along with Caterpillar, Detroit Diesel, and Cummins, have brought about a major reduction in diesel exhaust emissions. Equally important is the fact that the technical advances introduced to meet these tighter emission standards have been transparent to the end-user “” the coach enthusiast. The bottom line is that the level of performance demanded by those who prefer diesel power has not been compromised.
When it comes to the diesel engine, the popular image many of us used to have is a throwback to the 1980s when the Motor City gallantly tried to introduce diesel-powered personal-use vehicles, which failed to catch on. Unfortunately, these vehicles, along with old city transit buses and 18-wheelers, are remembered for their unsightly belching of black exhaust, the odors they emitted, the inability of diesel-powered personal-use vehicles to perform adequately, and the annoying noises that all of them produced.
Having said that, the diesel engine offers several important advantages in vehicle engine applications. First, diesel fuel contains almost 30 percent more energy than gasoline. Second, in many places, diesel fuel is less expensive than gasoline (in some places it costs significantly less). And third, because of its heavy-duty construction, a diesel engine that is properly maintained offers a significantly longer operating life than does a gasoline engine. Equally important is the fact that in the past 10 years diesel engine technology has dramatically changed for the good, mitigating many of the issues that had long been the downside to the diesel engine. Underscoring these achievements are the successful efforts by Cummins, Caterpillar, and Detroit Diesel to build diesel engines that yield greatly reduced levels of NOx (oxides of nitrogen), non-methane hydrocarbons, and particulate matter (soot). An additional benefit is that today’s diesel engine compartments also yield far less noise.
October 2002 saw a mandate imposed by the EPA that set diesel emission standards for NOx and non-methane hydrocarbons at a cycle level of 2.5 grams per brake horsepower-hour. The standard for particulate matter remained unchanged for now at 0.1 gram per brake horsepower-hour. If these standards are exceeded, a certain dollar amount of penalty per engine is levied against the engine builder. However, the EPA does allow the diesel emissions controls’ strategies to be temporarily adjusted by the engine’s electronics to provide improved overall engine performance under certain operating conditions, such as cold starting, engine protection at high altitude, or extremely hot temperatures. These strategies, known as Auxiliary Engine Control Devices, or AECDs, are reviewed and approved by the EPA to ensure that there is a negligible overall impact on exhaust emissions.
Beginning in 2007 diesel emissions standards will be further tightened, requiring an additional 50 percent reduction in NOx and 90 percent reduction in diesel particulate matter. Ultimately, the emission standards that will be in effect by the end of this decade are this: There will be no discernible differences between the gasoline and diesel internal combustion engines as they pertain to exhaust emissions.
So, what is NOx? What causes internal combustion engines to produce NOx? And what actions have Detroit Diesel, Caterpillar, and Cummins taken to reduce NOx levels?
First, it’s important to understand what NOx is: a chemical compound molecule that is created when nitrogen and oxygen atoms combine. NOx is the stuff that makes the atmosphere appear hazy (often referred to as smog). It is created in the engine cylinders by the high temperatures reached during the combustion process, temperatures that cause the oxygen and nitrogen in ambient air to combine. To reduce the formation of NOx, ignition or combustion must yield lower temperatures. In today’s diesel engine, this is being achieved in a couple of different ways.
One method involves a process called cooled exhaust gas recirculation (CEGR). Central to this process is a heat exchanger, through which a specific amount of exhaust gas is taken from the exhaust system and passed along one side of the device. Coolant from the engine’s cooling system is passed along the other side of the heat exchanger to cool the exhaust gas. Engine electronics determine on a real-time basis the amount of cooled exhaust gas that needs to be recirculated back into the cylinder along with ambient air and injected diesel fuel. The amount of cooled exhaust gas continuously changes as performance requirements change “” during acceleration, climbing hills, variations in ambient temperature or humidity, or steady highway operation. In addition, variable-geometry turbochargers, which are also governed by engine electronics, adjust to changing performance demands to ensure that the amount of air being packed into the engine for combustion is correct.
The cooled exhaust gas that is put back into the cylinder is inert. Simply put, this means it won’t combust. By mixing inert gas (exhaust), fuel, and ambient air together prior to the mixture being ignited in the cylinder, the higher temperatures that create NOx are reduced.
Another way to reduce high temperatures in the cylinder is through variable valve timing, new fuel injectors, and engine electronics. The engine’s electronics manage when, how often, and in what amounts the fuel will be injected into the cylinders. The injectors deliver highly atomized molecules of fuel that are uniformly surrounded by the ambient air, a condition that yields a more complete and cleaner ignition of the molecules of fuel. One of the results is less particulate matter.
These new fuel systems are able to inject fuel at higher pressures not just once but multiple times during each combustion cycle. By having more than one fuel injection “” sometimes referred to as pilot injections “” delivered to each cylinder during the compression stroke, ignition of the air-fuel mixture is spread out (not just one big bang), resulting in lower combustion temperatures. Using pilot injection to spread out the initial ignition of the air-fuel mixture also reduces the harsh knock associated with older diesel engines.
One challenge chassis builders have faced in accommodating diesel engines with the new emissions systems is the higher heat loads placed on the engine cooling systems, requiring them to build larger and more efficient systems. Increasing the capacity of the engine cooling system has required larger radiators and also necessitated that larger fans be added to generate greater airflow. This means that larger hydraulic pumps are needed to drive the larger cooling fans. In addition, the configuration of the cooling airflow path has been fine-tuned to more efficiently manage the airflow.
As mentioned, the year 2007 represents the next milestone for reducing diesel engine emissions. With changes in the industry rapidly being paced by new and evolving technologies, what are understood today as being state-of-the-art solutions to reduce diesel exhaust emissions may be supplanted with new or more refined ones. Therefore, it’s understandable why the companies that build diesel engines are reticent about revealing in detail how they intend to meet the new and much tighter emissions standards scheduled for 2007. However, there are several technologies that will no doubt play an important role in the future.
For example, once thought to be an impossible goal to achieve, diesel fuel with a sulfur content of 15 ppm (a standard that is required to be phased in beginning in 2006) is already available in the Los Angeles market. Reducing the sulfur content from the current 300 ppm down to 15 ppm is significant, because the sulfur found in petroleum poisons the precious metals in the after-treatment devices (catalytic converters) that will be needed to glean the remaining NOx from diesel exhaust later on this decade. Low-sulfur diesel fuel is also important for the future of after-treatment particulate filters/traps.
Exhaust system particulate matter filters/traps is another technology undergoing intense research and development. These are devices that remove soot from diesel exhaust. Various approaches are being developed to rejuvenate these traps/filters, which is necessary to prevent them from clogging up. One possible method is to have the engine electronics regulate and time the injection of small amounts of diesel fuel into the trap where it is then ignited, turning the soot into ash that is removed from the filter during regular maintenance intervals.
Some engine builders are already using an exhaust system device called an oxidation catalyst. This device takes the non-methane hydrocarbons in diesel exhaust (another emission that needs to be reduced) and chemically converts them into harmless water and carbon dioxide. The next step is to develop devices that will chemically convert NOx into harmless gases. One method will be to develop new after-treatment catalysts similar to the oxidation catalysts used for hydrocarbons. One such method may include urea injection, where the urea chemically reacts with NOx in the catalyst system. The future may see combinations of all three of these systems (particulate filter, oxidation catalyst, and NOx catalyst/after-treatments) being incorporated into diesel vehicle exhaust systems.
In the past decade, the diesel engine has gone from being a “bad boy” to one that is poised to play an even more important role in the overall mix of vehicle propulsion systems. One need only look at how well diesel technology has been applied to personal-use vehicles in Europe, where 50 percent of all personal-use vehicles are diesel-powered. And as a sign of things to come, in the fall of 2004 Jeep will begin offering diesel-powered SUVs (the Liberty) for sale in North America. Many, including EPA and CARB, are beginning to accept the fact that the diesel engine can be a friend to the environment.