Common Rail Invasion

By Richard McCuistian. Get Ready for a New Wave in Light-Duty Truck Diesels for the 21st Century.  Rudolph Diesel’s 1895 engine design used compressed air to blow coal dust into his combustion chambers, and the fact that his design used cheaper fuel and not as much of it caused diesels to rapidly become the only real choice for stationary or ship engines.  

The one major drawback diesels had was their inability to reach high rotational speed; the compressed air diesel fuel system was extremely slow and crude, not to mention the fact that the air pump was huge. But with the expanding interest in diesel engines worldwide, it didn’t take too long for Robert Bosch to solve the problem. By mid-1923, Bosch had developed a dozen or so basic designs for injection pumps and was testing his new hardware on diesel engines. Two years later, he had chosen the best design, and in 1927, the first production units were leaving his factory.

It was a watershed breakthrough for the diesel engine. It brought diesels up to more useful speeds (today’s smaller diesels spin at more than 5,000 rpm). Because the injection pump was a lot less bulky than the old air pumps, diesels became an available choice for vehicular propulsion, and the diesel engine began to evolve and spread rapidly. Other manufacturers began making injector pumps based on the Bosch design.

One Bosch publication chronicles a Bosch injection pump-powered diesel vehicle that set a land speed record of 224 miles per hour, all the while averaging more than 17 miles per gallon. Show me a gas-burner or a hybrid that will even go that fast, let alone get that kind of fuel economy at that speed.

The Bosch diesel fuel injection pump has to be totally fuel-bound (no air bubbles) with 14 to 22 psi of internal pump pressure, and, depending on the design, it uses cams and plungers to send powerful pulses of fuel to specially designed injectors. Every injector feed line has to be exactly the same length. Furthermore, the inevitable bends in the lines that snake to the injectors can’t be tighter than about 2 inches, and even the clamps that hold the lines are specially spaced and placed by fuel system engineers. Because the timed pulse and its resulting “pop” of the injectors is based on the pressure wave traveling through the lines at the speed of sound, a shorter or longer line will cause a misfire due to altered injector timing.

Early light-duty vehicle injection pumps used cable-, vacuum- or wax pellet-operated cold-start timing advance mechanisms and glow plugs or intake air heaters for warming up.Turbocharged diesels generally have a module added to the pump to modify fuel delivery and timing while boost is under way, as well as a solenoid to stop the fuel from flowing at key off.

Different Strokes

Ford and General Motors (GM) already had vehicular diesels on the road in the early 1980s while Dodge pickups remained diesel-less and bounced around from one prospective vendor to another before deciding to go with Cummins in 1989. It wasn’t a bad choice: Cummins makes some of the most dependable diesels on the planet (without glow plugs, no less), but the Dodge Cummins fuel system has gone through numerous changes since the automaker’s first diesel pickup rattled across a dealer’s lot. The turbo was a must from the get-go for the Dodge diesel, but an intercooler had to be added because the 1989 to 1991.5 powerplant tended to overheat in a full throttle, hard pull situation. This was due to the inherent heat of turbo-compressed air.

Interestingly, the intercooler didn’t add any horsepower or torque because boost was decreased from 25 psi to 18; it simply helped the engine run cooler. As a matter of fact, Dodge Cummins horsepower remained constant until 1996 – 160 for automatics, 175 for manuals – when boost pressures were raised slightly to 19 for automatics and 25 for manuals. The resulting automatic transmission horsepower was bumped up to 180 and manual horsepower to 215, as a result of the higher boost pressures used on that platform.

GM’s 6.2L was a lackluster performer from the start, and its 6.5L successor didn’t do much better. But let’s give the engine its due: There are a lot of GM diesel lovers out there, and the 6.5L still pushes a lot of AM General Hummers up and down American (and Iraqi) highways.

Ford and International have had a successful marriage since 1983: first with the 6.9L, then the 7.3L that came along in 1987 and finally, with the birth of the Power Stroke, everything diesel began to change, first in the minds of engineers, and then everywhere else. Driven by tightening emissions standards, injector pumps became a thing of the past under the hood of a Ford after 1994. However, GM waited a half-dozen years before putting a smart box in control of their 6.6L Duramax and with it a totally redesigned fuel system courtesy of Bosch and Isuzu and a variable geometry turbocharger. The latter was something Ford wouldn’t incorporate until 2003 when the 7.3L was replaced by the International Truck and Engine Corp. 6.0L.

Dodge replaced their distributor-type injection pump with an inline P7100 in 1995 and then reverted to the VP44 Electronic Injection Pump in 1998. But the automaker didn’t go totally electronic until 2003, and when it finally did turn loose of the high-pressure pulse pumps, it was to go common rail. However, Dodge pickups won’t experience the magic of variable geometry turbochargers (VGT) until the Cummins 6.7L comes out in 2007. By the way, the Cummins VG Turbo is an ingenious design that has only one moving part besides the turbine/compressor shaft, and in my opinion it will probably prove to be a lot more dependable over time than the GM and Ford VGT designs.

COMMON RAIL, VGT AND EGR

I think of the Ford/International 6.0L platform as a “bridge” because it spans the gap between the 7.3L with electric-over-hydraulic injectors and the coming 6.4L common rail engine. Common rail injection, made possible by today’s powerful on-board microprocessors, will be the industry standard for a long time to come. The fuel rail pressure, ranging from 5,000 to more than 20,000 psi, is controlled by the electronic control module (ECM). The operation of the injectors is very simple: They still pop, but high fuel pressure on top of the injector piston prevents it until the ECM or fuel injection control module (FICM) sends about 90 volts to operate the injector solenoid, which opens a pressure bleed in the chamber above the piston.

Because the upper chamber has more area than the pintle chamber, bleeding the pressure off the upper chamber creates a pressure differential that allows the fuel pressure at the needle valve to open the pintle at the injector tip. One fairly prevalent trend on common rail diesels is to route incoming fuel through the module (ECM or FICM) that operates the injectors, which cools the electronics. Cummins does it on their medium-duty truck engines, and Chevy does it on the Duramax.

Let’s talk about that VGT. For a while, I thought the VGT was doing its most intense work with the vanes wide open. After all, that’s when most of the exhaust is passing through the vanes, right? Well, yes, but actually, the turbine is spinning its fastest when the vanes are more to the closed position because the exhaust is forced to move faster and so does the turbine, which is joined at the hip with the impeller.

Think of how much faster water goes out of your garden hose when your finger is partially covering the opening. Faster exhaust gas flow means a faster spin on the turbine; it’s as simple as that. Remember that these turbos are inherently noisier than the older-style models; don’t throw one at a customer’s recently purchased used truck because his 7.3L was quieter.

GM’s VGT operates pretty much the same way: driven open and closed by an oil piston similar to Ford’s. Actually GM had the VGT first, and its turbo added a vane position sensor for extra feedback. The vanes are used by GM and Ford alike to create exhaust backpressure (EBP) for faster cab warm-up and to help facilitate cooled EGR flow, which became necessary with tighter NOX standards.

Oddly enough, Ford uses a GM-style EGR control for its 6.0L exhaust gas recirculation (EGR) valve, while GM hung a pair of old-fashioned Ford-style exhaust gas recirculation control (EGRC) and exhaust gas recirculation vent (EGRV) solenoids on the Duramax, basically delivering vacuum from an old-fashioned, belt-driven pump to the EGR diaphragm. This older Ford-style EGR system design would be replaced under the Duramax hood by a more robust electronic EGR unit in 2005.

Both Power Stroke and Duramax initially used an electronically controlled throttle plate and a mass airflow (MAF) sensor to determine when the plate angle should be changed, as well as a boost sensor for pressure feedback to assist in EGR flow. On later models, the throttle plate was discarded and replaced with a simpler concept. Ford moved to an EGR “scoop” and the VGT exhaust backpressure-producing effect to force EGR flow. Using cheap fuel will “coke up” the EGR valve and intake with heavy carbon deposits, but using a couple tanks of really good diesel fuel will it clean up, according to some anonymous lab guys at Ford. Those same guys said that much of the fuel at U.S. pumps is far too dirty.

Incidentally, GM’s 2004.5-and-later EGR valve uses a powerful electronic stepper motor pushing against a spring-loaded valve stem. The stepper motor can’t close the valve, it can only shove it open, and it bumps the stem three times at Key-On to make sure the powertrain control module (PCM) knows where the stem’s closed position is.

6.0L’S TROUBLESOME MIDDLEMAN

Ford/International’s Caterpillar-style high-pressure oil systems have always been problematic; indeed, most of the 7.3L engines I worked on in the field had high-pressure oil system concerns of some kind, and the complexity of the hydraulic electronic unit injectors (HEUI) was at least a part of the problem. Details are sketchy, but it appears to be a large part of the driving force behind the common rail system that’s coming in 2007. There was just too much that could go wrong with that high-pressure oil system acting as the middleman between the PCM and the fuel spray. One article I read said biodiesel isn’t good for these systems either, so you might consider that before you pump it into one of these vehicles.

The 2003 6.0L came with whole host of different concerns, beginning with surge-causing injection control pressure (ICP) sensors and fuel-seeping injectors that would dribble disturbing amounts of fuel into the crankcase, thereby destroying the turbocharger bearings. Another surge concern – more “heard” than “felt” in many cases – could be caused by a sluggishly responding VGT actuator piston. If disconnecting the EBP sensor (a major PCM feedback for controlling the turbo) eliminates the surge, the VGT solenoid/piston assembly can be renewed without replacing the whole turbo. The newest design VGT control valve has a 200-micron screen to prevent contaminants from fouling the valve.

The 6.0L high-pressure oil supply is carried to the oil rail by a disposable rolled steel standpipe that might split open and dump high oil pressure; you get a new one with a set of head gaskets. The original swash plate-style high-pressure pump turned out to be more trouble than it was worth, giving over to a piston-style pump with four pistons. The 2004 model ICP sensor was moved to the front of the right oil rail, which was a great idea.

Oddly enough, the 2005 Excursion held onto the old style swash plate pump and the EGR throttle plate/MAF sensor setup that disappeared from the pickups and vans beginning in that model year.

The high-pressure oil pump and the oil cooler are neatly nestled in the valley between the heads, with the pump at the rear of the engine and the oil cooler at the front. The timing and pump drive gears are mounted at the rear of the engine in a sealed splash-oiled chamber just in front of the flywheel. The oil cooler and/or the high-pressure oil pump chambers can leak engine oil, and sometimes you have to dump some dye in the oil, plug in the block heater and apply shop air pressure to the oil gallery to ferret out the source of the leak.

COMPARISONS

I found a 2005 Duramax sitting right next to a 2005 Power Stroke on my friend’s lot the other day and drove them both. The Duramax felt a bit smoother and more ergonomic than the Ford. But that 6.0L is mighty hard to beat when it gets into its power curve, and in its stock configuration it would run off and leave the Duramax after about 20 mph.

As emissions standards tighten, technology improves and diesels continue to evolve, we can probably expect to see smaller and smaller vehicular diesels that burn cleaner and get even better fuel economy than the hybrids. It wasn’t uncommon for an early 1980s diesel Ford Escort or VW rabbit to get 50 miles out of a gallon of fuel. There aren’t many hybrids around that can do that yet.