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Diesel Evolution
NOTE to reader: This article gets more advanced as it progresses, so be prepared to stretch your understanding! Richard
Diesel engines rely on tremendous heat created by high compression to fire their mix. For ever pound of compression, you get about 2 degrees Fahrenheit of temperature, so with a diesel engine producing about 450 lbs of compression you have about 900 degrees of temperature there, and when you spray fuel into that superheated air, you get a burn that lasts all the way to the bottom. That's what gives a diesel engine so much torque. A gas engine's combustion event is over with right after it happens, so a piston on a gas burner is coasting during the majority of its stroke. Imagine yourself turning a crank by slapping it. That's what a gas burner does. A diesel is more like pumping the crank the way you would do it if you really needed to produce some heavy torque.
A diesel piston comes to within .040 inch of the the head, which is flat, but there is a combustion chamber of sorts built into the top of the piston and it looks something like a deep bowl in the center of the piston... VW diesels had five different head gasket thicknesses for the purpose of fine tuning the clearance between piston and head.
Diesels have to inject their fuel at extremely high pressure (1800-3500 lbs) through the nozzles on the injectors - that pressure is produced on older diesels by mechanical pumps that send pulses of fuel to specially designed injectors that react to the pressure by delivering a spray of fuel when the pressure pulse hits the injector. The timing of the injection event has to be precise and usually takes place right at or slightly after the piston reaches Top Dead Center (TDC) on the compression stroke.
Rudolph Diesel’s 1895 design used compressed air to blow diesel 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 feed 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 (smaller diesels spin at over 5000 rpm nowadays), and since 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 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 over 17 miles per gallon! Show me a gas burner (or a hybrid) that’ll 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 14-22 lbs of internal pump pressure, and (depending on the design) it uses cams and slots and stuff 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 two inches, and even the clamps that hold the lines are specially spaced and placed by fuel system engineers. Since 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 vehicle injection pumps used cable, vacuum, or wax pellet operated cold start timing advance mechanisms and glow plugs or intake air heaters warming up. Turbocharged diesels generally have a module (see illustration) added to the pump to modify fuel delivery and timing whilst boost is under way and a solenoid to stop the fuel from flowing at key off. Different Strokes – LT Diesel Progression
Ford and GM already had vehicular diesels on the road in the early eighties while Dodge pickups remained dieselless 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 first Dodge 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 89-91.5 power plant tended to overheat in a full throttle hard pull situation 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 AT, 175 for MT) when boost pressures were raised slightly (to 19 for AT, 25 for MT), and the resulting Automatic Transmission horsepower was bumped up to 180. MT horses = 215 due to the higher boost pressures used on the MT platform.
GM’s 6.2 was a lackluster performer from the start, and its 6.5L successor didn’t do much better, but let’s give it 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 7.3 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, but GM waited a half-dozen years before putting a smart box in control of their 6.6L Duramax rattler, and with it a totally redesigned fuel system courtesy of Bosch and Isuzu and a variable geometry turbocharger, something Ford wouldn’t get until 2003 when the 7.3L was replaced by the International Truck and Engine 6.0L.
Dodge replaced their distributor type injection pump with an inline P7100 in 1995, then reverted to the VP44 Electronic Injection Pump in 1998, but didn’t go whole hog electronic until 2003, and when they finally did turn loose of the high pressure pulse pumps, it was to go common rail, but Dodge pickups wouldn’t experience the magic of variable geometry turbochargers until the Cummins 6.7L comes out in 2007, a year still future at this writing. 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 seemed to span 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 onboard microprocessors, will be the industry standard for a long time to come; the fuel rail pressure (usually ranging from 5000 to more than 20,000 lbs) is controlled by the ECM and 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 FICM sends about 90 volts to operate the injector solenoid, which opens a bleed above the chamber above the piston.
Since the upper chamber has more area than the pintle chamber, bleeding the pressure off the upper chamber creates a pressure differential which allowsthe fuel pressure at the needle valve to open the pintle at the injector tip. One trend that seems to be fairly prevalent on common rail diesels is to route incoming fuel through the module (ECM or FICM) that operates the injectors so as to cool the electronics. Cummins does it on their medium truck engines and Chevy does it on the Duramax.
| Trivia: Duramax and the Dodge Cummins can call their box an ECM because it only controls engine operation, not the whole powertrain. The transmission is controlled by another box, sometimes with inputs of its own, and sometimes with CAN or SCI bussed inputs relayed from the ECM. |
Variable Geometry Turbos Let’s talk about that VGT.
For a while, I thought the variable geometry turbo charger 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, yeah, 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 old style, so don’t go throwing 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 like Ford’s (actually GM had it first) but GM’s turbo added a vane position sensor for extra feedback. The vanes are used by GM and Ford alike to create exhaust backpressure for faster cab warm-up and to help facilitate cooled EGR flow, which became necessary due to tighter NOX standards. EGR Choices
Oddly enough, Ford uses a GM-style EGR control for their 6.0L Exhaust Gas Recirc valve, while GM hung a pair of old-fashioned Ford-style (EGRC and EGRV) solenoids on the Duramax, basically delivering vacuum from an old-fashioned belt-driven pump to the EGR diaphragm. That archaic 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 MAF sensor (to determine when the plate angle should be changed) and a boost sensor (for pressure feedback) to assist in EGR flow but that crazy throttle plate was discarded and replaced with a simpler concept on later models.
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 clean up that mess, 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/up 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 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 HEUI injectors was at least a part of the problem. Details are sketchy, but that would appear 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 read said bio diesel isn’t good for these babies either, so you might consider that before you pump it into one.
The 2003 6.0L came with whole host of different concerns, beginning with surge-causing ICP sensors and fuel-seeping injectors that would dribble disturbing amounts of fuel into the crankcase, thereby destroying the turbo charger bearings. It was later determined that the 'split shot' injector function wasn't being handled well by the Siemens injectors and a reflash was engineered to remove that strategy, but the reflash caused some fuel economy issues.
Another surge concern (more heard than felt in many cases) could be caused by a slugglishly responding VGT actuator piston, and 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 stand pipe that might split open and dump high oil pressure (you get a new one with a set of head gaskets), and 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 4 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. Tricks and Stuff A no-start due to a lost cam signal (the sensor is really long and passes through the block down behind the power steering pump) can be caused by a lost camshaft peg. That peg produces the signal as it whips past the VRS cam sensor and if it his flown its coop, well, you might have to replace the camshaft. If the sensor isn’t damaged and checks out okay, then peer into the hole with a mirror and a light while somebody bars the engine over to see if you see a hole instead of a peg.
A crazy rough idle can be caused by a Transmission Range sensor reading something other than the truth, so pay attention to that PID if you’re looking for a concern like that.
A worn out thrust bearing can cause the crankshaft to move enough so that the 60-2 tooth pulse ring moves out of line with the crank sensor, and that can cause weird misfire conditions, so be aware of that as well. Conclusions 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 eighties diesel Escort or VW rabbit (I owned a few of those) to get 50 miles out of a gallon of fuel. There aren’t many hybrids around that can do that yet!
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