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April 22, 2005

Common Rail Fuel Injection

The Latest Diesel Engine Innovation Provides More Power, Economy and Lower Emissions

During the winter in south Florida, the sailfish fleet lines up along the reef in hopes of intercepting the fish as they migrate through the area. In the past, you could always tell when one of the big diesel boats hooked up. A large black smoke cloud would appear as the captain either hit the throttles to take up slack or began furiously backing down on the fish. We always took it as a good sign: "Hey, look. So-and-so is hooked up!" But the joy of catching a fish often gave way to coughing fits as the occupants of the hooked-up boat inevitably breathed in the noxious fumes.

Diesel engines hate idling, at least the older models did. Sailfishing often entails hours of idling with abrupt bursts of throttle that dump fuel into the cylinders, resulting in that black cloud which comes from unburned fuel. Fuel delivery systems in diesels remained virtually unchanged for decades, but during the past 10 years, rapid technological improvements have brought welcome changes to the marketplace, making diesels cleaner, quieter and more efficient. The latest development comes from common rail fuel injection technology.

Older diesels used several different fuel injection systems. Perhaps the most common utilized a series of rocker arms driven off of the camshaft lobes, one rocker per cylinder, which directed fuel to the individual injectors. The spring-loaded injectors remained closed until they reached a certain pressure. They would then open and dump their load of fuel into the cylinder all at once.

This mechanically driven method obviously worked, but it gave better results on the top end of the power curve than it did at lower speeds. That's because traditional diesel fuel injection systems optimized fuel delivery for rated speed conditions, basically wide-open throttle. Consequently, these engines often ended up pouring in too much fuel at low speeds or at the wrong time. As a result, the fuel didn't burn completely, causing smoky and inefficient operation. That pre-existing condition often led to problems for engines that idled a lot, like the ones our sailfishing friends used, and cylinder life suffered.

Common rail fuel injection eliminates the mechanical connection and makes injection an electronic process, in essence separating the two functions of generating pressure and injecting the fuel. Many factors influence torque, fuel consumption, emissions and noise in diesel engines, but the most significant ones include fuel pressure as it enters the cylinders, the shape of the injection and the number of injections per cycle. These qualities are far better with common rail than with previous technologies.

In a common rail injection system, a central accumulator rail stores fuel under very high pressure, as high as 36,000 psi. The pressurized fuel flows through high-pressure lines from the rail to the individual electronically controlled injectors under the direction of the engine's computer (ECU). This system ensures very high pressures right to the injector tip, even at low speeds. The higher pressure atomizes the fuel as it enters the cylinder, creating a fine fuel mist that burns cleaner and more completely. And because the computer precisely controls the injection, fuel can be introduced more than once in each power stroke.

These microbursts spread out the combustion process over a very short time, contributing to a more complete burn. This greatly reduces noise since the explosion that occurs as the fuel is compressed now actually consists of a series of smaller explosions. "Before, you had one big bang that made a lot of noise," explained John Wheatley, president of Florida Marine Tech of Riviera Beach, Florida, the Cummins and MAN dealer for the Palm Beach area. "In a common rail engine, you've got a series of smaller bangs as the fuel ignites, and that makes them much quieter. And since you've eliminated the rocker arms riding on the camshaft, you've also eliminated a lot of gear train noise."

Cummins illustrated that point at last year's Miami Boat Show with a display in its booth that featured audio recordings of two diesel engines, a traditional mechanically injected engine and a common rail injected engine. You pressed a button to play the recordings of each engine at idle, and the distinct difference between the two became immediately apparent. But you truly begin to appreciate the difference when you spend the day fishing on a boat equipped with common rail engines. Smoke never appears, throttle response improves noticeably, and if you happen to be the guy paying the refueling bill at the end of the day, you get to keep a few more of those hard-earned bucks riding on your hip instead of enriching the local marina.




More Bang Per Stroke
Different companies utilize common rail technology in slightly different ways, but they all have the common thread of introducing fuel more than once per injection cycle. Some engines use two bursts, while others use three. The general consensus is that more is better. Common rail engines will soon have as many as five separate injections per cycle, making those engines even cleaner and more efficient.  This should allow companies to meet the stringent Tier 2 EPA emissions standards that take effect for pleasure craft in 2006. Tier 2 requirements took effect for commercial engines in 2004. 

"The accuracy of the fuel delivery is very good, which allows us to meet emissions standards while also providing better fuel consumption numbers," says Jim Kahlenbeck, director of Worldwide Cummins MerCruiser Diesel Engineering. "Using three fuel bursts enables us to substantially reduce emissions of nitrous oxide, hydrocarbons and soot. It's a much softer burn." Kahlenbeck also points out that common rail injection enables manufacturers to optimize fuel delivery for every speed point in the power curve, which helps prolong cylinder life.

"Everything depends on how the mapping is done in the computer," adds Doug Rose, product manager for Volvo Penta. "With the ECU precisely controlling timing, we can consistently control fuel metering and flow, so power is optimized." With this much control, horsepower ratings on a given block increase with little penalty, further improving power-to-weight ratios, traditionally an Achilles' heel of diesel power.

Common rail injection also benefits engine design. "The adoption of common rail fuel injection eliminates the unit injection pumps, which previously protruded into the area between the V of the cylinder banks," says Dr. Gerd-Michael Wolters, a board member of MTU responsible for engineering. "This allowed us to carry out a substantial redesign of the crankcase. Positioning the engine charge manifolds and engine coolant flow channels in the V not only utilizes the space released by the injection pumps, but also by combining these components in a boxlike structure attached to the cylinder banks, we were able to brace the banks against each other. This adds stiffness to the crankcase that greatly reduces deformation of the cylinder liners and, hence, plays a vital role in minimizing lube oil consumption and cylinder wear."

Expect to see a lot more common rail diesel engines in the future. Tier 3 EPA standards loom down the road, and engines need to get even cleaner to pass these strict controls. That's good news for us as consumers and good news for the environment, but it will definitely be a little harder to tell when the boat down the line from you has hooked a sailfish.