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How Diesel Got Its Groove Back

Posted on 8/25/2016 11:33:39 AM

We’re talking about the modern era Tier 4 Final diesel engines that use high pressure common rail fuel systems and electronically actuated injectors governed by a electronic control module (ECM).

These systems can put small, rapid-fire bursts of fuel into the cylinders at rates in some cases of more than 6,000 times a minute. As a result, today’s big diesel engines can produce more power with less fuel than ever before while simultaneously cutting exhaust emissions by more than 95 percent.

To give you a better idea of how these modern miracles of engineering work we talked with Jim Fier, vice president of engineering at Cummins and Ilidio Serra, manager technical service support, Robert Bosch automotive aftermarket division.

We start by looking at the difference between the older style injectors and today’s new technology.

Mechanical Injection
Prior to modern emissions regulations most diesel engines relied on mechanical fuel injection – a camshaft lobe pushing against a roller tappet drove a plunger that Maintenance Sidebarpressurized the fuel. In these systems, pressurized fuel travels through a line until it hits a spring on the injector and forces it open, allowing the fuel to flow into the cylinder. Pressures up to 15,000 psi were possible, but only one injection per rotation of the cam lobe and one shot of fuel per combustion cycle were possible.

Mechanical fuel injection is simple and reliable. It’s still used on lower horsepower engines, but it can’t provide the precise control, emissions reductions and broad power range needed for today’s larger Tier 4 Final engines, primarily those 74 horsepower and up.

As emissions regulations grew increasingly strict, improvements evolved, including distributor pumps, in-line pumps and unit injectors, eventually taking pressures up to 23,000 to 26,000 psi. Many OEMs were able to get through the Tier 3 emissions requirements using these more sophisticated systems. But the real miracle didn’t happen until the introduction of high pressure common rail fuel systems (HPCR), which enabled injection pressures up to as much as 36,000 psi.

Common Rail System

Maintenance Sidebar 2In a HPCR system, the injectors draw their fuel from a single accumulator-like rail that serves all the injectors with a common source of fuel. The fuel stored in the common rail is pressurized, up to 30,000+ psi, while waiting to be used.

The advantage here is that you are no longer depending on a cam lobe or fuel pump to pressurize the fuel at the injector. The tasks of pressurization and injection, which are linked in mechanical systems, become independent. And the higher the pressure, the better the fuel atomizes once it is sprayed into the cylinder.

Instead of the speed of the cam or the fuel pump determining when the injector opens and closes, an HPCR system controls the injector with a small, rapid firing actuator, either a solenoid or piezo crystal built into the injector. And because they are electronically actuated, they can fire as fast as you can turn an electric currrent on and off.

These electronically governed injectors provide much better control of injection timing and quantity compared to mechanical systems, says Fier. “This has been a significant enabler of designing cleaner and more fuel efficient diesel engines,” he says.

Multiple Injection Events

“Combustion in a diesel engine is much like the recipe for baking a cake,” says Serra. “If you measure your ingredients correctly, have the right temperature settings and time, you get a perfect cake every time.”

The challenge is that the recipe may change from one second to the next. Every time you drop into a different gear, climb a hill or pour on the throttle to maximize breakout force, the mix of pressures, temperatures, injection events and timing changes the recipe.

Only common rail systems with ECM brains and ultra-fast, electronically-controlled injectors have the speed and versatility to respond to these changes and still maintain emissions parameters, fuel economy and power output.

Diesel Dynamics

A cylinder in a gasoline engine will consume one injection of fuel within 40 to 60 degrees of crankshaft rotation. A diesel engine burn lasts much longer, from 90 to 120 degrees, says Serra. This slow, expanding explosion is what gives diesel engines their stump-pulling torque. Shaping and maximizing the efficiency of this combustion plume is of paramount importance.

Valve placement, piston bowl shape and the design of the injector tip all influence how the plume circulates within the cylinder, Serra says. But injector timing and frequency are two elements that can change as demands on the engine change.

In a typical low-power HPCR combustion scenario you might have three injection events, in sequence as follows:

It starts with a small, quick pilot injection to get things going. During light and medium engine loading early pilot injections also help control the formation of NOx (a pollutant regulated by Tier 4 Final) and reduce noise – that unmistakable diesel engine “knocking” sound at idle.

Next comes a full load, main injection for power. Six to eight events are possible to modify combustion or assist emissions aftertreatment.

Finally you get a small, post injection to flare off any unburned fuel remaining in the cylinder. Post injections also control particulates in the exhaust, provide extra energy for aftertreatment systems and reduce turbocharger lag.

When the application demands high power, the ECM will usually order one long injection.

Speed Freaks

Engineers measure the speed of these injection events in microseconds, which is 1/1,000th of a second. There is a window of approximately 7,000 microseconds for all the injections to occur, during which time:

The injector solenoid or piezo crystal actuator begins to open within 100 to 150 microseconds of being energized.

With a three-injection event, each injection delivers a measured quantity of fuel at approximately 1,225 times per minute at idle (750 rpm) and up to 3,300 times per minute at rated speed (2,200 rpm).

In a six-injection event each injector can deliver bursts of fuel at up to 6,600 times a minute at 2,200 rpm.

After the injection event, it takes another 50 to 100 microseconds for the solenoid or piezo crystal actuator to return to rest and dissipate any electrical charge.

Computer Control

“The on-engine electronic control module manages all aspects of the fuel system control,” says Fier. “The ECM not only contains the electronics needed to actuate control valves and injectors, but also contains the engine calibration and diagnostics. It is basically the brain of the engine,” he says.

And while the hardware in most HPCR fuel systems may be similar, the electronic logic used to drive the system can be an important differentiator between the performance of different engines, Fier says. Engine calibration and electronic management have become more complex and must be fully integrated with air handling, fuel systems, aftertreatment and filtration, he says.

Injector Tips

Each injector has one nozzle with an array of spray holes that are optimized to meet power requirements as well as emissions performance, says Fier. Nozzles are steel and use different heat treatment methods to withstand high operating temperatures.

As emissions requirements have become more stringent, the nozzles’ ability to deliver a consistent and specific spray of fuel to the cylinder has become more critical, Fier says. The injection nozzle is an integral part of shaping the plume at the moment of combustion. The nozzle spray holes are matched to the cylinder bowl to get the best atomization of fuel and thus the best power density, lowest emissions and reduced fuel consumption, he says.

Clean Fuel

While the materials used for injector tips haven’t changed much in the evolution from mechanical to electronic injection, the injectors in HPCR systems are still vulnerable to contaminated fuel, says Serra. “Dirt, especially hard quartz particles, turns the fuel system into a very efficient hydro-grinder and will shorten the life of the fuel system and engine,” he says.

When you hear people preach about the virtues of clean diesel and good filtration, this is why. Even water in the fuel, at 30 to 36,000 psi and 5,000 to 6,000 times a minute, can greatly accelerate injector tip wear.

Safety

While the fuel in the common rail is under extreme pressure, the main risk to mechanics working on a system is when the engine is running, as most engines depressurize the fuel system within seconds of shutting down. Nonetheless you should always follow OEM recommended procedures when bleeding off or working on fuel systems.

“The new engines require technicians to forget their old diagnostic habits such as opening fuel lines on a running engine,” says Serra. “The older systems only pumped 0.01 ounces of fuel per firing stroke, per cylinder at full load. Therefore, the most fuel you would get out of a single fuel line was approximately 10 ounces of fuel at minimal pressure after a minute.

“With a common rail engine, doing the same thing would generate almost one gallon of fuel at significant atomization,” says Serra. “The velocity of the fuel within a few inches of the leak is high enough to penetrate the skin or gloves,” he says.

Diagnostics

Some of today’s engines can have hundreds of different fault codes for various conditions and symptoms, but fault codes don’t always solve a problem. “Even with all these fault codes, diagnosis still requires a well-trained technician who uses a systematic approach to diagnosing an engine system,” says Serra. “There is no replacement for experience and an understanding of cause and effect on the engine. For example, a misfire fault code can be caused by not only a defective injector, but by a faulty EGR system, valve adjustment or wiring harness system.”

The hardest problems for technicians to diagnose are the no-fault-code related complaints, Serra says. Unless they understand how the whole system is supposed to behave, what normal data looks like and how to approach diagnosis, they will be lost, says Serra.

“With the older mechanically injected engines, 95 percent of the fuel system was contained between the injection pump and the injectors, so diagnosis was fairly easy,” says Serra. “On a common rail engine, the fuel pump and injectors are only 25 percent of the fuel system. “I have seen cases where a technician has spent weeks on a modern engine by not following the diagnostic process, replacing many expensive components only to find out he missed a simple fault such as a plugged fuel filter.”

Durability

According to Fier, a recent tear-down and inspection of a Cummins Tier 4 engine showed that a 20,000 hour life-to-overhaul could be expected on its HPCR injectors. The caveat is that it depends on duty cycle, application, good filter maintenance and clean fuel.

“These engines do not require a scheduled change of fuel injectors at mid-life and are expected to achieve the same life as the engine,” Fier says. “Perhaps more important than life in hours is the total number of injections over the life of the HPCR system, with 1 billion injections being a typical number.”

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