Clean biodiesel fuels, dual fuel CNG/diesel injection systems and similar innovations are helping to clean up diesel emissions—and so are microprocessors. The improvements made in the last few years to light-duty diesel engines are very impressive. Automobile technicians sometimes tend to avoid servicing diesels, but it’s time to reconsider; today’s diesel technology is fascinating, and a deeper understanding could prove beneficial.
You may recall from an earlier TechNews Clean Diesels article that a diesel mega-boom is forecast.
The demand is driven by the need for better fuel economy, without sacrificing power and vehicle space. Before long we’ll be seeing more light-duty diesel passenger vehicles, SUVs/crossovers and light trucks. All this, thanks to improved engine design, improved air management systems (variable rate turbos), more sophisticated fuel injection (FI) systems, after-treatment devices (for NOx and particulates) and electronic controls that have reinvented the once noisy-stinky diesel. Here, we’ll look into one aspect of the diesel’s improved performance—the quiet diesel evolution. Diesels are so improved they’re now being accepted “uptown” by discriminating luxury car owners who’ve decided to “buy in.”
A Bit of Theory
Recall that gasoline engines are spark ignited (SI). Usually gasoline is ingested with air into engine cylinders with a relatively low compression ratio (~9:1) to avoid preignition and possible detonation (knock) of fuel during the compression stroke. Because the diesel takes in air alone, it can tolerate much higher compression ratios (23.5:1 and higher) to achieve “more bang for the buck.” Air is compressed until “white hot” (Charles’ Law), so it’s hot enough to auto-ignite the fuel when it arrives. Fuel injection pressure must be high enough to overcome compression, for proper mixing, and correct compression-ignition timing. But injection is not instantaneous; “injection lag”—the time required to build injection pressure and get it to the nozzle and “pop” it open—delays things. Once fuel is injected, “ignition lag” also occurs.
Injector design and spray pattern, compression temperature, fuel cetane rating, and more influence ignition lag.
The “lag” during the compression stroke tends to cause diesel fuel to ignite spontaneously, thus creating the familiar diesel knock or “clatter” sound; upon ignition, cylinder pressure suddenly rises and “rings” the cylinder. In gasoline SI engines, such pressure spikes lead to destruction of pistons, rings, and bearings, but the diesel engine is more robust. Today, to reduce the annoying rattle (along with emissions), engineers have smoothed the combustion process. Much as a basketball is best dribbled with a smooth downward motion of the hand and wrist (rather than by slapping it), a piston should move smoothly from its upward compression stroke to its downward power stroke. Smooth up-down piston transition requires not only precise timing of fuel injection, but also of the delivery rate, for differing speed and load conditions.
Traditionally, light and medium duty diesel engines for motor vehicles have used “indirect injection” (IDI). With IDI, fuel is injected into a red-hot pre-combustion chamber (“pre-cup”) or a swirl chamber for less noisy combustion. IDI engines use a “throttling” pintle-type nozzle to introduce a very fine “pilot-injection” of fuel for less abrupt ignition. Once ignition starts in the pre-cup, the rate of fuel delivery is increased, and a rich expanding/swirling flame is then “blown” through an orifice into the main chamber to drive the piston downward. Delivery of fuel then tapers off until combustion can no longer be sustained in the excess of air (as much as a 40:1 ratio at idle), making the IDI diesel a relatively quiet and economical diesel engine—but still not acceptable to many American motorists, or for the EPA.
High injection pressures are needed to rapidly swirl and mix the fuel-air mixture for complete homogenizing and burning of fuel for economy, low emissions and less combustion noise. Yet peak injection line pressures (before the nozzle opens) for IDI are limited— some in-line pump injection systems could climb to near 12,000 psi; smaller engines using distributor type (rotary) pumps would run lower pressures.
Both in-line and rotary pumps use individual high-pressure (injection) lines to deliver fuel to nozzleholder-assemblies (NHA) on the cylinder head, but such long injection lines radiate undesired noise and add to injection lag, and parts sometimes suffered from cavitation erosion.
Solving Noise Problems
To overcome these limitations, today’s direct injection (DI) systems have much shorter injection lines, if any at all. Plus, the fuel injection process is much more precise to quiet the “clatter.” Much like electronic Pulse Width Modulation (PWM) electronically shapes voltage, fuel injection timing and delivery is “rateshaped”— hydraulically speaking— to allow softer/quieter combustion.
Fuel is now delivered in pulses:
Main-injection of fuel must be delivered directly onto (or into) the piston in relatively few degrees of crankshaft rotation. In short: deliver more fuel, in less time, with higher injection pressures. The older mechanical pump/governor + injection lines + nozzle holder assembly systems cannot meet today’s peak Touareg’s V-10 engine) for ultra-efficient injection and combustion of fuel. Left unmanaged, such pressures would radically increase ignition/combustion noise; electronics has taken over.
Thanks to high-speed microprocessors, OEMs have achieved remarkable improvements to meet and exceed demands for quiet, clean engine power. In 1982 Volkswagen(and others) introduced EDC (electronic diesel control) to manage their TDI (Turbo-Direct Injection) diesel engine/injection system. Today Common-Rail (CR) and Unit Injection (UI) systems are likewise microprocessor controlled to manage “rate shaped” delivery to individual cylinders. Fuel is dynamically delivered using varying rates of multiple injections (up to 5 or more) in fractions of a crankshaft degree!
The result is drastically smoothed idle rpm and roughness; far less combustion noise; no smoke, less odor and emissions; and improved fuel.
Thanks to technology, most people standing next to a newer idling diesel car cannot tell it from its gasoline counterpart.
Helping to make today’s diesels even more clean and quiet while offering 20 percent better mileage are new “piezo” style injectors which no longer rely on mechanical or solenoid actuation.
Piezo common-rail and unit injectors offer even more improved rate-phasing characteristics (they’re more than four times faster responding) for each injection cycle. According to Denso, pilot spray intervals as small as 0.1 milliseconds are possible, and even more impressive designs are on the way. According to Bosch, we can expect to see a piezo common-rail injector with two nozzle needles and two sets of spray holes; finer-sized holes are used for pilot-injection, and larger holes for main-injection.
Reportedly, we can expect to see even further reduction of noise and up to 70% lower emissions than presently possible.
Little wonder then that technical engine improvements, the late 2006 arrival of ultra-low sulfur diesel (ULSD) fuel, and the growing demand for more power and vehicle space with improved economy all are contributing to experts’ predictions for a U.S. diesel mega-boom.
Yes, diesels (once again) are coming. And to make things even more exciting, perhaps before long we’ll see a few diesel hybrids!
Killing the Clatter… Prior Efforts
Various mechanical designs have been tried to reduce diesel ignition clatter:
Killing the Clatter… Today’s Technology
Two types of fuel injection systems have emerged:
|“V10- TDI” (left) gives it away – it’s a turbocharged 10 cylinder direct-injected diesel.|
|You cant see much of Touareg’s engine (left) unless surrounding under hood sound baffles are removed. The engine uses 10 combined Pumpe-Düse (pumpnozzle) camshaft-actuated unit injectors. Duration of injection is electronically controlled.|
|Volkswagen’s 10-cylinder Touareg SUV (left) has plenty of power, without the noise.|
Source: Rob Rodriguez (ASE)