Modern Diesel Technology — written by Ralph Birnbaum
In spite of their native torque and superior fuel efficiency, diesel engines spew some pretty toxic emissions that contain high levels of particulate matter (PM), oxides of nitrogen, sulfur, some HC and CO, and a slew of other malodorous nasties that contribute to smog and respiratory distress. In fact, one of the big problems with diesel exhaust is that the soot particles killing us may be too small to see. To paraphrase an old saying: “Out of sight, out of breath.”
Clean Diesel Fuel
In this article, we'll talk about some new and very interesting breakthroughs in diesel engine technology. But before we do that, we need to talk about changes in diesel fuel, because none of the new technology would work without Ultra Low Sulfur Diesel.
Engine oil is critical in new diesels. For 2007 model year vehicles with DPFs you'll need to use API CJ-4 engine oils. This oil has low ash content, which is critical, since regeneration will not clean away ash accumulations inside the diesel particulate filter. If enough ash accumulates to cause a serious exhaust restriction, the DPF must be removed and cleaned or replaced. CJ-4 is generally backwards compatible with older diesels.
Traditional middle distillate petroleum diesel fuel chemistry has long contributed to increased harmful emission levels. Even Low Sulfur Diesel has enough sulfur at 500 ppm to quickly contaminate diesel exhaust scrubbing devices like catalysts and exhaust filters. Before anyone could get really serious about cleaning diesel exhaust, we had to clean the fuel.
Beginning January 1, 2007, the 2007 Highway Diesel Rule mandates a 50% reduction in NOx and HC, and a 90% reduction in PM, over 2004 levels. To make this all possible, diesel fuel has undergone a really radical change that reduces sulfur content by 97 percent, from 500 ppm -- to only 15 ppm! Even without changes in diesel engine design or the addition of special emission control devices, the new Ultra Low Sulfur Diesel (ULSD) reduces harmful emissions immediately; and more importantly, it no longer contaminates diesel emission scrubbing devices and filters.
Hardware changes are already on the road in newly designed diesels from Cummins, GM, and Ford. Hot on the heels of these blue-collar diesels are passenger car diesels from Mercedes-Benz, VW/Audi, and a new Honda diesel for the Accord.
Fuel dispenser stickers distinguish Low Sulfur Diesel (LSD) from the new Ultra Low Sulfur Diesel (ULSD) at the pump, and from non-Highway diesel (don't get caught using that stuff in your tank!). Use of LSD can damage diesel emission components designed to work with ULSD. Component damage may include catalyst poisoning and soot clogging of a large exhaust filter referred to as the Diesel Particulate Filter (DPF). More on that in a bit.
The 2007i Duramax Components and Technology
GM's LMM Duramax is a great example of new emission control devices that make diesels stronger and cleaner. Let's look at key features:
• A common rail fuel system operates at extremely high pressures. Common rail is like a port-fuel gas system in some ways, since all injectors are fed through a common connecting fuel supply tube called a rail. The big difference is that common rail pressures are hundreds of times greater than those found in gas port injection. The Duramax? The 2007i common rail operates at 26,000 psi. (Pressures like that will blow your gas engine fuel pressure gauge to smithereens, so be warned!)
• New Duramax injectors are graded during the manufacturing process and matched as sets for uniform delivery. Note the laser etched numbers on the head of this example. image GM
Exact injector characteristics are programmed into the PCM for more precise fuel control. The injectors in the LMM have also been improved to work with special pulse timing injector operation used during exhaust cleaning.
• A throttle valve has been added to the air intake, positioned ahead of the air intake heater. - image GM
Yeah, we know, that seems to defeat the entire purpose of a free breathing diesel, but the throttling blade is closed selectively to improve EGR flow and limit intake air during an exhaust cleaning process called regeneration, another topic we'll get to in more detail before we're through.
• EGR? No getting away from EGR these days. The LMM has a new, larger EGR cooler to pull some of the intense heat from the exhaust, increasing exhaust gas density.
The LMM exhaust system is a real piece of work, and looks like it might weigh more than an Aveo.- image GM
Main exhaust components include: a Diesel Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), and a conventional muffler. The exhaust can get hot enough to brand you like a longhorn, especially during regeneration, so the tailpipe is fitted with a specially shaped venturi cooler tube to suck cool, fresh air over the exhaust metal exterior. In spite of this, please give a hot diesel exhaust a wide berth.
• The Diesel Oxidation Catalyst (DOC)is similar to those used for years on heavy-duty off-road diesels. Like an automotive gas engine catalyst, the DOC has a washcoated honeycomb containing precious metals that promote both oxidation and reduction.
• Diesel Particulate Filter (DPF) - The inside of the DPF is made of semi-porous walls formed into long tubes. Half the tubes are closed at the exhaust inlet, the other half are closed at the outlet. Exhaust gases enter the open end of the tubes and must travel through the tube walls to exit. PM captured by the semi-porous materials in the tubes must eventually be burned off in a process called regeneration. Unlike the DOC, the DPF is a pretty big change from business as usual. While it contains catalyzing agents, its main function is to capture exhaust PM – by filtration.
Catalyst efficiency is monitored by an exhaust gas temperature sensor mounted between the DOC and Diesel Particulate Filter (DPF). - image GM
Regeneration: Works Like a Self-Cleaning Oven
Regeneration works a lot like a self-cleaning oven: Exhaust is heated into a small blast furnace and accumulated particulate matter is incinerated.
How often is regeneration needed? That depends on several factors:
Distance driven since the last regeneration
Fuel consumed since the last regeneration
Engine run time since the last regeneration
Exhaust system backpressure measured as the differential pressure across the DPF
Exhaust backpressure is monitored by a differential pressure sensor. The sensor is connected to two pressure tubes: one at the DPF inlet, the other, at its outlet. - image GM
The differential pressure sensor sends a voltage signal to the PCM that indicates exhaust restriction caused by PM accumulations.
If the vehicle is driven hard and loaded on a regular basis, the exhaust will self-clean more often, due to the high exhaust temperatures. If the vehicle is not driven hard enough to generate high exhaust heat, the soot will accumulate faster.
Passive or Active
If the engine is forced to work hard enough to get the exhaust good and hot, Passive regeneration takes place. Vehicles towing loads at highway speeds undergo passive regeneration.
If the PCM calculations indicate that the vehicle isn't getting enough passive cleaning to burn off PM accumulations, it starts a cleaning process called Active regeneration. The active cleaning occurs only while the vehicle is driven at speeds greater than 30 mph for 20-30 minutes. During this time, the exhaust temperature reaches 500 degrees C (1022 degrees F). If the cleaning is interrupted for any reason, it picks up where it started on the next drive cycle that meets the necessary operating conditions.
If the vehicle is never driven in such a way that regeneration can complete, a warning light illuminates on the dash to alert the driver that a regeneration is needed.
What Happens During an Active Regeneration?
To get the exhaust hot enough to clean the DPF:
The PCM adds post injection fuel pulses; dumping extra fuel into the catalyst generates almost half the heat needed for soot incineration.
At the same time, the throttle valve limits air intake volume, increasing engine temperature.
The Intake Air Heater and the variable turbocharger may also be used to increase exhaust heat under some circumstances.
A second exhaust gas temperature sensor located after the DPF monitors the regeneration process to assure that the exhaust is hot enough for cleaning, but not so hot that it damages the exhaust or poses a danger to the vehicle or its occupants.
Computer Controlled Injection
New exhaust scrubbers catalyze toxic gases and filter out particle emissions (soot). To increase engine power, improve fuel efficiency, and lower emissions to manageable levels, engineers have also looked for ways to improve what happens inside the cylinder on the power stroke. The fairly recent addition of computer controls to diesels lets them benefit from the same precise fuel monitoring and combustion event timing used in gasoline engines for years. Diesel computers now control injection pressure, the length of time each injector is open (its pulsewidth), and the exact timing of each fuel spray.
This is a huge departure from the mechanical hydraulic diesel injection pump and poppet valve injectors. Remember the old manual “advance knob” used to adjust injection pump timing? Long gone. Instead, each modern diesel injector spray is measured in milliseconds (or even microseconds!), and timed to arrive within a fraction of a degree of a target crankshaft position. Injection pressure is just as important to fuel control, and many modern diesel controllers can adjust fuel pressure over a wide range, on the fly.
Let's look at evolving diesel technology, and some of the systems and components in use.
Direct Injection (DI) – Direct Ignition sprays highly pressurized fuel directly into the cylinders. The old prechamber design is eliminated. DI has other advantages over Indirect Injection, including more power and cleaner and more efficient burning of each precious drop of fuel. The early disadvantage with DI was that a single shot of fuel pumped straight in to the cylinder makes a big bang, and an annoyingly loud engine.
Both mechanical and hydraulic force have been used to increase injection pressure. Elecrtronically-controlled Unit Injectors use a camshaft operated piston. HEUI injectors use hydraulic oil pressure, but dependability has been an issue.
Hydraulic Electrically-actuated Unit Injection (HEUI) – HEUI generates high injection pressures using hydraulic force from a high-pressure lube oil pump. Injector solenoids are then triggered by a high voltage signal from the PCM, or from a separate injection control module. The firing duration of either mechanical or hydraulic unit injectors is controlled by a computer-controlled electrical solenoid. It takes a lot of electrical power to operate the solenoids in the presence of such high-pressures, so expect unit injection solenoid initial voltages of 50-100 volts DC, depending on injector design.
The force multiplication needed for high pressure injection is generated inside the injectors by an intermediate piston located above the fuel plunger. The intermediate piston is larger than the plunger and “amplifies” the oil pressure. It is commonly referred to as an intensifier piston, as a result.
HEUI injectors commonly spray at 20,000+ psi. Such high pressures require extra caution, since they can cause serious injury, or worse.
Common Rail Fuel Systems
The diesel Common Rail is very similar to the fuel rail found in gasoline engines; a common fuel loop supplies pressurized fuel to all injectors, simultaneously. One big difference between the gas and diesel versions of common rail is rail pressure, which is far higher in a diesel.
Common rail diesel pressures are generally 20 to 240+ times greater than those found in a high-pressure gas engine fuel loop operating at 50-100 psi. So far, common rail diesel systems operate at pressures as high as 24,000+ psi: no one knows what the ceiling will be on system pressure.
Here's a video showing the main components of a common rail injector and how it uses pilot injection. There are a few typos in the captions, but the animations are pretty cool.
Common rail diesel fuel injection systems share other characteristics with common rail designs found in gasoline engines. Computer-triggered, solenoid-operated injectors, one per cylinder, are connected to a common fuel rail accumulator that is pressurized by a central pump. The computer controls injection timing and duration, and continually adjusts fuel rail pressure, independent of engine speed.
Animation of a Bosch Common Rail pressure pump.
Maximum rail pressure gets all the headlines, but it's important to remember that common rail pressure can also be regulated and adjusted over a wide range, from 2000 to over 20,000+ psi, independent of engine rpm. The rail also acts as an accumulator to help maintain the desired rail pressure, even as the injectors open and spray. The engine computer sends a command to the high-pressure pump, and then it monitors the actual fuel pressure in the rail to verify that the corrections are made. This closed loop monitoring can be viewed in serial data using the correct scan tool interface.
Common rail fuel systems have some important advantages. They give engineers a lot of design flexibility to improve engine performance and economy, while lowering emissions and noise. The reason this works so well is that the common rail injector can be controlled very precisely, allowing for fuel to be delivered in several pulses with great timing accuracy.
Any fuel system leak is a serious matter in a system operating over 20,000 psi.
Pilot Injection - The introduction of pilot injection solves the pressure spike problem for DI by injecting fuel directly into the cylinder in several smaller pulses. This allows modern DI engines to run quieter and smoother, with improved fuel efficiency and power, and fewer harmful tailpipe emissions. In pilot injection, a small spray of fuel initiates the combustion process, followed by an additional pulse (or pulses) that keep the “fire” burning in steps. This stepped injection happens in a fraction of a second, far too fast for old style hydraulic/mechanical injectors. Multiple explosions eliminate the “big bang” power stroke pressure spike that made DI injection so noisy. In fact, pilot injection can be configured to make a diesel run as quietly as a gas engine.
New injectors combine pilot injection and special spray patterns to maximize combustion efficiency and reduce emissions.
Before pilot injection technology was introduced, each diesel cylinder received all of its fuel in a single shot. This “single slug of fuel” approach has been the standard for decades, but it has some serious limitations. First, it takes time for that single shot of fuel to mix with enough oxygen to support combustion. Even then, combustion is less complete, especially at higher rpm when there is less actual time for combustion to occur.
When the power stroke burn does start, a rapid rise in cylinder pressure from a single, large explosion causes the diesel noise we all know well. The high-pressure spike also contributes to an increase in NOx formation. Noise and NOx are not desirable in modern diesels.
The idea behind pilot injection is to divide fuel delivery over several rapid fire pulses. The first pulse delivers a few cubic millimeters of highly atomized fuel that mixes quickly with air inside the cylinder and burns fast. Multiple additional fuel pulses are then added to continue the burn: reducing noise, extending the power stroke duration, and lowering emissions.
Replacing “Slowenoids” with Piezo Injectors
Pilot injection pulses must occur very rapidly; so rapidly that the duration of each event is measured in milliseconds. Injectors must have very fast reflexes to open and close that fast. To further improve injector response and accuracy, relatively “slow-acting” solenoids will soon be replaced with fast-acting actuators containing electrically activated piezo (pronounced pie-eee-zoh) crystals. Piezo crystals expand when an electric field is applied to them, moving the injector needle to release pressurized fuel to the injector nozzle. To shut off a piezo injector, the controller simply reverses the voltage polarity of the control signal.
Piezo injectors are much faster acting than solenoids. The speed of piezo injectors allows more injection pulses to be added to the pilot injection sequence, further reducing diesel clatter. They are already used in some applications. Expect these injectors to be the new standard, replacing solenoid operated injectors altogether.
From the Bosch website: To control the injection valves, current Common Rail injectors use a rapid-action actuator employing piezo crystals. The piezo crystals expand within an electrical field, switching five times as quickly as a solenoid. Bosch has built the actuator into the body of the injector. The movement of the piezo package is transmitted non-mechanically and therefore, without friction to the rapidly switching nozzle needle. This doubles the injector's switching speed, allowing a more precise measurement of the amount of fuel injected, leading to a reduction in emissions.
By adding a post combustion pulse, the combustion event is extended and the added fuel helps reduce NOx.
Four Valves per Cylinder
Multiple valves have long been a part of gasoline engine design, and are now favored in many modern diesels as well. The four-valve design improves air flow, promotes combustion chamber swirl, and reduces noise. Just as importantly, it allows the injector to be placed in the center of the valves where multiple orifice injector nozzles create a symmetrical, concentric spray pattern for uniform fuel distribution. This allows more even burning, increased power, and fewer harmful emissions.
Combine these new technologies with the cleaner diesel fuels now being sold, and diesels meet 50 state emissions standards; this makes them an increasingly attractive alternative to hybrids. Expect a wave of new diesels to arrive soon, many in passenger vehicles.
To watch a video of Accord's 2.2L i-CTDI engine, click here.
The Honda diesel adds dual intake runners with a control valve that modulates air flow in one of the tubes to promote swirl. This highly professional video also displays many of the components already discussed here: four valves per cylinder, common rail configuration, and pilot injection. (By the way, 1600 bar common rail pressure equals 23,206.038 psi.)
Clearly, the modern diesel has graduated from simple mechanical injection into a vehicle-wide system designed to improve engine power and fuel efficiency, while reducing harmful exhaust gases and particulates. The modern diesel technician needs hi-tech skills and equipment to properly service and repair these systems.