What is a “modern” diesel, and is it really that much different?
There’s a lot of talk these days about “modern” diesels, and how today’s diesels have changed. The modern diesel is described as powerful, clean, quiet, responsive, economical, and smoke-free, but what actually constitutes a modern diesel? Generally speaking, a diesel must incorporate five basic features to be considered modern. Those five features are: (1), four valves per cylinder; (2), direct fuel injection; (3), computer controlled fuel management; (4), common-rail fuel injection; and (5), a “pilot injection” noise suppression cycle. You may notice that a turbocharger wasn’t included. That is because virtually all automotive diesels manufactured in America since ’94 are turbocharged, so a turbocharger is assumed to be part of the basic diesel design. Let’s look at each of these defining characteristics one at a time.
First, by using a cylinder head design with four valves per cylinder (two intake valves and two exhaust valves) maximum airflow can be obtained for a given bore diameter with less shrouding (flow restriction) by the cylinder wall. The cylinder head ports can also be aligned to promote maximum swirl within the cylinder for high combustion efficiency. Most importantly, a four-valve configuration allows the fuel injection nozzle to be positioned in the center of the combustion chamber for a symmetric fuel injection pattern directly down the centerline of the cylinder. This arrangement provides the most uniform fuel dispersion for combustion efficiency with a minimum of exhaust pollutants. Earlier designs with only two valves per cylinder did not allow such advantageous injector placement.
Second, in recent past history, many automotive diesel engines were designed with indirect injection (IDI). IDI used a precombustion chamber. Fuel was injected into the prechamber where compression ignition occurred. The burning mixture then expanded through a passage or throat into the cylinder. This arrangement could be designed to produce high swirl in the cylinder, but the main purpose was to reduce engine noise. The clatter of pre-modern direct injection (DI) diesels is the result of a rapid pressure spike when the main injection pulse occurs. The resultant combustion produces the pressure spike that generates the noise. By initiating combustion in a much smaller precombustion chamber, the clatter wasn’t nearly as loud. Unfortunately, IDI diesels are not nearly as efficient or as clean as DI diesels, so all modern diesels are direct injection. The noise problem was solved with “pilot injection”, which we’ll cover later.
Direct injection diesels are as much as 15 percent more efficient than IDI diesels in terms of thermal efficiency. Power and torque output is up to 40 percent better with DI. Additionally, fuel consumption is up to 30 percent less, and that means carbon dioxide and hydrocarbon emissions are reduced by a similar amount. The transition of the burning gases from the precombustion chamber to the cylinder in IDI diesels results in significant heat losses into the cylinder head and cylinder walls, which reduces efficiency.
The third item that distinguishes a modern diesel is computer controlled fuel management. What this really means is computerized electronic control of the fuel injection pulses in response to fuel throttle position, engine speed and load, and feedback from numerous engine sensors. A computer can control the fuel injectors, pump pressure, and duration of the fuel injection pulse (called pulse width) with the speed necessary to permit real time combustion control. It also permits multiple injection events for each cylinder firing. Computer control provides the precision to properly time these events to meet both today’s and tomorrow’s emissions and fuel economy requirements. For example, some diesel fuel injection manufacturers are talking about the necessity to have as many as four or five separate injection events per combustion cycle. If you do the math for such a system on a V8 running 4000 RPM, that would be 80,000 injection events per minute! Only a computer is capable of controlling such a system with accurate timing. Of course, the injection system must also be up to the task.
The fourth requirement of a modern diesel is common-rail fuel injection. In order to allow the computer to control the timing, pulse width, and fuel pressure of the injection system, the injection system must be able to maintain pressure independently from engine speed, and to achieve injection pressures adequate for efficient direct injection. Currently, common-rail injection systems provide this needed flexibility. Operating independently of engine speed, the common-rail system can supply fuel pressure from 2000 to 24,000+ PSI to a common fuel rail that supplies each bank of injectors. The high pressure allows the use of injectors with multiple small holes for efficient distribution and atomization of the fuel pulse into the cylinder. Ultra high pressure creates small fuel droplet size and high injection velocity to promote complete burning of the fuel. This provides maximum power and minimal pollution. The high fuel pressure also allows the delivery of a programmed amount of fuel in a very short time. This is especially important to accommodate multiple injection events during each combustion cycle.
The fifth requirement of a modern diesel is pilot injection. Pilot injection is used on direct injection diesels to dramatically reduce noise, especially at low engine speeds and idle. Pilot injection is featured on ’01-03 Chevy/GM 6.6L DuraMax diesels, on ’03 Cummins 5.9L diesels for automotive applications, and on ’03 Ford 6.0L Power Stroke diesels. Pilot injection is the introduction of a small amount of fuel that starts combustion immediately prior to the main, power-producing fuel pulse. This smoothes the start of combustion, eliminating the pressure spikes that produce the clatter common to previous diesels, especially at idle. Done properly, a diesel with pilot injection will idle as quietly as a gasoline engine. Pilot injection also improves the cold start capability of diesels to equal that of gasoline engines at temperatures as low as -40º F. Additionally, pilot injection can help reduce the oxides of nitrogen from a diesel by lowering peak combustion temperatures.
Timing of the pilot injection pulse and the amount of fuel injected is critical to noise reduction. The time between the pilot pulse and the main injection pulse needs to be extremely short. Some injection systems are now capable of reducing this interval between pulses to as little as .0007-second. The result is combustion starts on a small scale that doesn’t generate noticeable clatter, and then builds gently, but quickly. This reduces noise, produces less vibration, and quieter cold start warm-ups.
Modern diesels have come a long way in just the last few years. They are no longer noisy and smoky, emissions have been significantly reduced, power and economy are up, and performance is astounding. However, the evolution of the diesel is far from over. Many other technologies are being tried and tested, such as cooled exhaust gas recirculation, cold-burn post injection, closed-loop ion-sensing for real time combustion control, and piezoelectric injection. Most of these developments are aimed at further reducing diesel emissions, and most have a positive effect on power and economy. Low sulfur fuel has been mandated by 2006, which will eliminate any remaining diesel odor and permit the use of catalytic converters to reduce oxides of nitrogen. Self-cleaning particulate traps are coming to remove the last vestiges of smoke from diesel exhaust too. We’re also likely to see a reduction in diesel engine weight and increases in maximum engine speed. The modern diesel is already incredible. One can only wonder what future development will bring. One thing is for sure, you’ll be seeing a lot more exciting diesel-powered vehicles in the future. One of them might even be yours.