Diesel engine technology?

[JB]

I haven't kept up very well with modern Diesel engine technology.<br /><br />I have owned four Diesel cars; three Mercedes and one Oldsmobile. The Olds doesn't really count because the engine was a converted gasoline engine and a disaster. I am a big fan of diesel economy and longevity, but the cars I had were real slugs.<br /><br />I have been reading about modern, DFI turboDiesels that are quiet, torquey and even more efficient than the old, prechambered, ambient aspirated engines I had.<br /><br />One term I have read that I don't understand is "common rail". Is that just another way to say DFI?? If not, what does it mean and why? Are they using electrically operated, computer controlled injectors rather than mechanical?<br /><br />I know that Daimler Chrysler sells the ML in other parts of the world with sophisticated Diesels and that they may include them in the next generation AAV, when low-sulphur fuel becomes mandatory (2006?). Does Skinnywater have anything to tell us about that?<br /><br />Bring me up to date guys.

 

[jimchere]

Re: Diesel engine technology?

JB,<br />Common rail is nothing new. Surprisingly, I pulled one of my old college ME textbooks and its included in the description of diesel fuel metering systems. There are 3 basic types of fuel metering for diesel engines:<br />1. Individual Pump Systems, a seperate metering and compression pump for each cylinder. Bulky and expensive.<br />2. Distributor systems, a single pump for compressing the fuel with a dividing device for distributing the fuel to all the cylinders.<br />3. Common Rail Systems, a single pump for compressing the fuel within a common manifold (or "rail") with a seperate metering element for each cylinder.<br />The Common Rail System is more precise and lends itself better to electronic control (metering elements, or injectors seperately controlled for each cylinder), increasing both efficiency and power while reducing emissions and improving fuel economy. This system is also used on modern gasoline engines. My '98 Jeep 4.0L gas motor utilizes a "common-rail" system and is a type of multi-port injection, and can also be called DFI.<br />To quote (source stated below, and I'm bored today): "Common rail systems were, once, quite popular for large, slow-speed diesel engines, but through the years, were replaced by jerk-pump common rail injection. In this system, the high pressure pump serves only to deliver fuel into the common rail, with the pressure held constant by a pressure-regulating valve (or varied by the throttle and governor, when desired). Thus the maximum pressure is under direct control, and the metering problem is not hnadled by the high pressure pump; hence extreme accuracy in manufacture is not demanded (lower costs). On the other hand, the discharge from the nozzles is regulated by the size of the metering orifice (and time) and the presure drop in the delivery lines. Hence the nozzles must be closely matched to ensure equal distribution among the cylinders. The common-rail system tensd to be self governing; if the speed falls, an increased quantity of fuel is injected (more time the individually controlled ejectors are open), since the supply (pressure) of fuel is indepenent of engine speed. Interest in the common rail system has revived with the demand for greater output (per cubic inch) and higher speeds."<br />Source: Internal Combustion Engines and Air Pollution, Edward F. Obert, Harper and Row Publishers, Inc. 1973, pg 439.

 

[JB]

Re: Diesel engine technology?

Thanks, Jim.<br /><br />I now understand "common rail" and the ability to computer control timing and duration of injection.<br /><br />All of my MB Diesels used a mechanical pump with individual cams and pistons for each cylinder. A real nightmare to service.<br /><br />My Diesels all were also prechambered, a small chamber in the head where fuel was injected and combustion started. I understand that many modern Diesels no longer use prechambers, but inject fuel directly into the cylinder. Both VW and Mercedes now use this technology, TDI by VW meaning "Turbocharged and Direct Injected" and CDI by Mercedes meaning about the same thing.<br /><br />What have Cummins, GM and International (Navistar) done in this area?

 

[Peter J Fraser]

Re: Diesel engine technology?

JB & Jim<br />Detroit Diesel had what you could call the forerunner to the modern common rail system running in their two stroke diesels from the mid 30's.<br />It used a small gear pump to supply fuel to a common gallery in the cylinder head at a pressure of around 60 psi. This gallery supplies fuel via steel pipes to the "unit injector" body which is placed in the centre of the combustion chamber. The fuel is atomised by a 3rd lobe on the cam via a rocker arm directly onto the top of the injector plunger. the movement down of the plunger traps the fuel below and pressurises it to the point where the delivery valve in the injector spray tip is opened and delivery into the chamber starts.<br />The start of the injection is timed mechanically by the length of the push rod so it may be advanced or retarded as desired . The actual qty of fuel delivered is controlled by a helix on the side ot the plunger inside which rotates to close or open the internal port that spills the surplus fuel flow back to the fuel tank. This surplus fuel is also used to cool the injector internals.<br />Cummins Diesel use a fairly similar system also with a "Unit injector". Both systems are very simple to maintain in service.<br />Detroit Diesel were the first to use a full electronic injection system (DDEC)which was released into service in the early 80's. <br />I have been involved in the heavy diesel industry for over 30 years and it is interesting watching the claims about the "new technology". Like all new ideas most, are just a revival of an old idea. <br />Some of the technology we see now is very good and has been made possible with the application of modern electronics.<br /><br />Good luck<br /><br />Peter

 

[DJ]

Re: Diesel engine technology?

JB,<br /><br />I'll also add that the common rail system allows for individual control (usually electronic-nowadays) of injectors as previously stated.<br /><br />Many diesels today use engine oil as the driver for the injector. Since extreme nozzle pressures are now the norm, engine oiil provided an easy medium to develop the intense hydraulic pressures needed. Once the high nozzle pressures are achieved, they are opened (or fired) electronically.

 

[mattttt25]

Re: Diesel engine technology?

you guys lost me on 90% of what you've talked about. but i will say that from what i've read, diesel technology in a marine application seems to be moving fast. more and more boats in the 20 to 28' size are coming with small, powerful diesel engines.