If you haven’t been watching for a while, it’s possible that almost everything you thought you knew about diesel technology has changed in the past few years. Of course, you already knew that diesel engines get better fuel economy than gasoline engines. And you also might know that the diesel produces much more low- to mid-range torque.
What might come as a surprise is that diesel engines now are almost as quiet and their emissions are as clean as many gasoline engines. You heard me right: no smoke, powerful and very quiet. Yes, times have changed and a literal renaissance has been taking place right beneath our noses. The automotive diesel concept has had a difficult time catching on in North America. While there was a time that diesel fuel was cheaper than gasoline in the U.S., those days are long gone.
In fact, we have just reached parity again after a long period where diesel was much more expensive than gasoline. This is in contrast to Europe, where diesel fuel is generally taxed at a lower rate than gasoline and has a greater appearance of value. This has precipitated a revolution with the majority of new cars in Europe being sold with diesel engines installed.
Then there are legacy issues with the technology itself. Diesel cars were, at one time, quite sluggish when compared to their gasoline-powered peers. This, combined with poor cold starting and pungent exhaust fumes, made them unattractive to the American consumer. But has the well been permanently poisoned?
Some experts say no, and in turn predict that the diesel’s current 5 percent market share will triple within the next six to eight years. It would seem that if the hearts of the American automotive consumer will be won over, it will be accomplished through the offering of superior products. Based on what is on the market right now, it appears that the diesel has a pretty good chance.
So how did the new diesels get so clean and quiet? There are a number of technologies that have contributed to the revolution. Let’s take a look at what makes today’s diesel engine different from anything we’ve known in the past.
Common Rail Injection
In the past, diesel fuel systems were mechanically controlled and injected fuel into the combustion chamber at relatively low pressures. Moreover, there was only one shot of fuel that would
be injected during a given combustion cycle. While we view them now as dinosaurs, these systems were reliable and were able to meet emission standards of the time.
However, today’s diesels are being held to the same emission standards as gasoline engines, and the diesel injection system plays a major role in meeting those standards. To that end, high-pressure common rail (HPCR) systems are now being used in virtually all automotive diesel engines.
PAGE 2Most HPCR injection systems use a three-piston pump that sends pressurized diesel fuel into a common rail. The common rail acts as a distribution point, feeding fuel to the individual injectors (one per cylinder) through steel lines. The injectors are electronically controlled and inject the high-pressure fuel directly into the combustion chamber when commanded open by a computer module.
HPCR looks very similar to gasoline direct injection (GDI) systems, except that the injection pressures are much higher. In fact, the newest systems are running at up to 29,000 psi. One might ask why the injection pressures are so high (and getting higher), when 3,000 psi used to do the trick. The answer is that higher pressures mean better atomization of the fuel and greater penetration into the combustion chamber. As the particles of fuel get smaller and better distributed, it is that much easier to surround them with air and burn them cleanly.
HPCR injectors are electronically controlled and can be turned on and off very rapidly. This level of control is so precise that now a fuel injection event can be broken up into five or more pulses during one combustion cycle. Here’s how it used to work. Only one large shot of fuel would be injected into the superheated air in the combustion chamber during a given cycle.
The injected fuel would take some time to absorb enough heat to ignite. During this ignition delay period, the fuel swirled and mixed with air. When the fuel finally did ignite, the majority of the charge already was mixed with air and burned very rapidly. This rapid burning resulted in a pressure spike in the combustion chamber, causing the characteristic knock that diesel engines were known for. This also generated significant NOx emissions, as combustion chamber temperatures rose to very high levels.
Current diesel systems using HPCR can start the fuel injection event with one or two very small shots of fuel known as pilot injection. Think of these as the kindling one would use to build a campfire. Pilot injection warms up the combustion chamber and makes it ready for the main injection event, which now ignites that much more quickly and burns smoother. This moderates the pressure rise in the cylinder and makes the engine much quieter, as well as limiting emissions.
Turbocharging
The days of the naturally-aspirated (non-turbocharged) diesel are over. Turbocharging is pretty much a no-brainer from a cost-benefit perspective, especially with diesel engines. With turbocharging, a smaller displacement engine can be tuned to deliver as much output as a bigger, heavier unit while getting better fuel economy. Today’s turbochargers are usually installed in combination with a charge air cooler (CAC), which further improves torque output, fuel efficiency, and emissions.
It gets better, though, because the latest turbochargers are designed to eliminate the age-old problem of turbo lag. Most new diesels are using variable geometry turbochargers (VGTs), where an adjustable nozzle is used to vary the velocity of the exhaust gases as they strike the blades on the turbine wheel.
At light loads when exhaust flow is low, the nozzle in the VGT can be closed to increase exhaust velocity and make the turbine spin up very quickly. This builds boost just as rapidly on the compressor side of the turbo and increases throttle response. When the engine is at full load, the nozzle in the VGT can be opened to control turbine top speed and boost pressure. When combined with electronic control, the VGT turns the diesel into a radically different driving experience.
PAGE 3Diesel Race Cars The top class of cars in LeMans racing is LMP1, which is where the latest prototype technology is showcased. The LMP1 class has been dominated in recent years by the diesel-powered entries of Audi and Peugeot. The diesel LMP1s have proven to generate superior torque, better fuel economy, and are unbelievably quiet. Beyond that, these cars do not generate any black smoke due to the use of diesel particulate filters. The 24 Hours of LeMans has been won by a diesel LMP1 every year since 2006. How’s that for a game-changer? |
In a quest for greater efficiency, some new diesels are using two turbochargers instead of one. The idea is that a small turbo is used to build boost rapidly at low speeds, where the large turbo can maintain high boost in full load situations. Where there would be two separate turbochargers in most designs of this type, the latest technology uses one turbocharger assembly with two compressor wheels on a single shaft. This dual-stage design eliminates weight and saves space, which is always a good thing.
Diesel Particulate Filters
The diesel engine of yesteryear was renowned for the black smoke it emitted when under load. For the diehard diesel fan, black smoke (also known as diesel particulate matter or PM) was considered to be a sign that the engine was making power. However, in time it became known that there were serious health issues associated with breathing diesel PM, and government authorities moved to have it limited through emission control regulations. The 2007 model year was where it really hit the wall: All on-road diesels (light and heavy duty) had to install diesel particulate filters (DPFs) in their exhaust systems in order to meet the new PM standards.
DPFs are known as an aftertreatment solution, because they deal with diesel particulate matter after it leaves the engine cylinders. While there are a number of different types of DPFs, the dominant technology utilizes a wall-flow technique. With this design, the substrate in the DPF is built very similar to an ordinary three-way catalyst used in gasoline engine applications.
The difference is that each passage is plugged at one end, with every other tube open to the inlet and the remainder open to the outlet. Exhaust gases that enter the open passages are forced to flow through the substrate wall to be able to exit at the other end. Diesel PM entrained in the exhaust is thus filtered by the DPF substrate, and the passages that are open to the inlet begin to fill up with soot and ash.
Obviously, there is a limit to the amount of PM that can be stored in a DPF. A plugged DPF will result in a major loss of power and could stop the engine from running at all. From time to time, diesel soot that is trapped in the substrate must be burned off in a process called regeneration. If a diesel engine is operated at high loads for extended periods, exhaust temperatures are high. Under these conditions, regeneration takes place by itself as the exhaust temperature remains high enough to light off the soot without any intervention on the part of the engine controls.
However, a diesel engine that is operated at light loads and/or short drive cycles will fill up the DPF quickly and may require active regeneration to clean the filter. Active regeneration usually involves injecting raw diesel fuel into the exhaust stream in order to increase exhaust temperatures and get a regeneration event started. This is performed in conjunction with a diesel oxidation catalyst (DOC) that is installed upstream from the DPF.
DPF Diagnosis and Service
Management of the regeneration process is critical, because the DPF substrate is not bulletproof. It is possible to crack or melt a DPF if regeneration temperatures become uneven or excessive. If you see black smoke coming from a 2007 or newer diesel vehicle, there is definitely a problem with the DPF and it will likely require replacement.
Another heads-up on DPF service: Not everything that is filtered by the DPF will burn during regeneration. Metal-based ash will collect in the filter over time, and eventually (hopefully after 100,000 miles or more), the DPF will have to be removed and cleaned or exchanged. A scan tool is the primary source of information on the health of the DPF.
Just Around the Corner
Urea Selective Catalytic Reduction (SCR) is here and you will be seeing it in your service bays soon. Mercedes Benz and BMW both have U.S. diesel offerings that use diesel exhaust fluid (DEF) and an SCR catalyst to meet NOx emissions standards. While the DEF tanks don’t have to be filled very often, the driver won’t ignore them, because the car won’t start if the tank is run dry.
The 2011 Ford Super Duty with the Ford-built 6.7 liter diesel also uses urea SCR; there is a blue DEF fill cap next to the diesel fuel filler. Yes, diesels are becoming quieter and cleaner, and this means updated maintenance procedures for the new vehicles that are entering your service bays.
