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Diesel Fuel Injector Performance: The Difference is in the Additives

By Keith Corkwell | November 13, 2008
Original equipment manufacturers want more power. Regulators want better emissions. End users want increased fuel economy. The role of fuels and additives in diesel engines is clearly being steered by several different demands.

At least two things can be accepted as fact. First, although no engine is 100 percent efficient, as with most mechanical equipment it runs best when it is clean. That is the way it was designed. Second, university, engine manufacturer, oil and additive company research repeatedly confirms that engines develop deposits over time. These deposits directly affect the engine's performance.

In gasoline engines, the air and fuel is pre-mixed before it enters the combustion chamber, which is why we worry about deposits in the fuel injectors, and on the intake ports and intake valves. However, in a diesel engine, the air for combustion enters the combustion chamber by itself. The fuel is injected directly into the cylinder, which is why the primary deposit concern in a diesel engine is the fuel injectors themselves.

A clean injector delivers a fine mist of tiny fuel droplets that are easily mixed with air, evaporated and burned. Dirty fuel injectors can reduce the amount of fuel delivered and result in a poor spray pattern with larger droplets. This makes it difficult for the fuel to mix, evaporate and combust. When some of the fuel fails to burn, three things can happen: power decreases, emissions increase because unburned fuel exits through the tailpipe, and fuel economy decreases. In other words, performance suffers.

Changes in Diesel Engine Design
To meet new emissions requirements and consumer demands, diesel engines are rapidly evolving and becoming more complex. These changes are happening worldwide in every segment of the diesel engine market, including off-road equipment and heavy-duty trucks in the U.S. market and passenger cars around the world. More than ever, diesel engine technology for all of these segments is looking more similar. While diesel engines have always been inherently efficient, meeting new emissions regulations requires tighter controls on primary emissions (NOx and particulate matter). This requirement is leading diesel engine designers to adopt some practices that have been on gasoline-powered engines over the years. Exhaust gas recirculation is used to reduce NOx emissions during combustion. Beyond the combustion process, aftertreatment devices are used to clean up the exhaust. In the case of diesel engines, this can include diesel particulate filters or selective catalytic reduction using urea injection.

Technologies that reduce emissions can be very effective, but they add cost to the engine and can reduce fuel economy. Therefore, additional resources are used to try to develop injection and combustion systems that produce maximum fuel economy and minimum emissions from the engine itself. One of the areas receiving a lot of attention is the fuel injection system. New systems are being designed to more accurately deliver the precise amount of fuel needed at exactly the right time in the smallest droplets possible.

The new systems rely on tighter tolerances, smaller injector orifices and higher pressures to produce smaller droplets. In older systems, all of the fuel for a combustion cycle was injected at one time. Today's modern systems allow several injections during a single combustion cycle. This allows engine calibrations to determine not only how much fuel is injected at each point in the cycle, but precisely when it is injected.


Diesel injector deposits can lead to power loss. After operating on a 10 percent biodiesel blend, this
engine experienced greater power loss than when diesel fuel was used alone. However, when a
detergent additive was added to the blend, the power loss was restored.

SOURCE: THE LUBRIZOL CORP.


While these injectors provide benefits on emissions and fuel economy when they are tested as designed, these changes can allow deposits to easily plug injectors. Some of the newer fuel systems can make this problem worse because they feature recirculating fuel systems, where a portion of the fuel that goes to the engine is returned to the fuel tank, hot. Heating the fuel can lead to thermal degradation and cause even more deposits. In other words, injectors are more likely to create deposits and, at the same time, injector deposits have a greater impact on vehicle performance.

Changes in Fuel
Engines are not changing in a vacuum. At the same time these engine design changes are occurring, diesel fuel is also evolving. To enable the use of some of the aftertreatment systems mentioned previously, it has been necessary to lower the sulfur levels in diesel fuel. While the new ultra-low sulfur diesel fuels allow the use of these aftertreatment systems in the exhaust, there is growing evidence that these new fuels may actually cause more deposits on the injectors in the engine.

The other major change in the market is the growing use of biodiesel. We know about the technical benefits of using biodiesel, such as improved cetane and increased lubricity. But what do we know about how biodiesel impacts diesel injector deposits? The answer is that biodiesel can be both better and worse than diesel fuels when it comes to forming deposits.

So far, available test data shows that biodiesel can actually help to reduce deposit buildup in some older engine designs that use indirect injection systems with lower temperatures and pressures. However, in more modern, hotter-running engines with direct injection systems, biodiesel fuel may lead to an increase in injector deposits. Ongoing research is trying to identify why some biodiesel fuel may cause this result.

We know that biodiesel can vary greatly depending upon its feedstock. With such a variety of biodiesel sources in the market today, it may take some time before we fully understand how these sources might impact deposit buildup in various engine designs. Even if the source of biodiesel is constant, some batches appear more prone to cause deposits than others.

The United States and Europe are two of the largest biodiesel markets in the world. Europe produces more than five times the amount of biodiesel produced in the United States. The use of biodiesel in Europe is even higher than that with some biodiesel produced in the United States and Asia being imported into Europe. While biodiesel is still relatively new in both markets, evidence is emerging that some biodiesel batches in Europe can cause injector deposits.

Injector Deposit Testing
While many of the factors previously mentioned are leading to increased attention to diesel injector deposits, these deposits are not completely new to the industry. In fact, the industry has dealt with these deposits and the performance issues they cause for many years. Because it is difficult to simulate the complex interactions that occur in a running diesel engine on a simple bench test, tests typically involve full-size diesel engines operating on dynamometer stands.

In the 1980s, two such tests were developed. In the United States, an injector deposit test based on the Cummins L-10 engine used direct injection but in an older, pressure-time mechanical injection system. The test was used for many years but has been out of production for more than five years. Meanwhile, in Europe in the 1980s a test was developed based on a Peugeot XUD-9, indirect-injection engine. While the test is still available today, new engines no longer use this injection system.


Oils with more saturated fat provide better stability, while oils with less fat provide better cold flow properties in biodiesel fuel.

SOURCE: NATIONAL RENEWABLE ENERGY LABORATORY


With today's diesel engines having deposit problems that lead to complaints of power loss, pressure has been on to develop a new test that represents these modern engines. The industry developed a new test this year, based on the Peugeot DW-10 engine. This turbo-charged engine features a common rail, direct, modern electronic injection system and represents modern diesel engines used in both passenger cars and heavy equipment.

Additive Solution
An additive solution is available for reversing the problems associated with loss of performance caused specifically by engine deposits. As diesel engine technology becomes more complex and tighter emissions standards are introduced, the use of diesel detergent additives appears to be growing around the world. Diesel injector additives are common in
Europe and have been for a number of years. In the United States, they are much less common but growing in popularity. Using detergent additives at low dosages in the fuel can control or even remove these deposits, restoring combustion performance.

To boost consumer confidence in biodiesel, detergent additives must prove that they prevent and remove deposits in a variety of fuels used in the market. This includes traditional diesel fuel and biodiesel blends from various sources. Proper additive treatment can address biodiesel performance concerns, including preventing or correcting field issues and improvements in the operability of biodiesel in modern vehicle technologies. Use of properly formulated additives can also reverse performance problems such as power loss.

The future is engines that are more likely to have injector deposits, fuels that are more likely to cause deposits and finally, a test that can measure the deposits. But, as the European market has proven, the proper additives can make the difference.

Keith Corkwell is with The Lubrizol Corp. Reach him at keith.corkwell@lubrizol.com or (440) 347-5963.
 

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