One of the latest advances in DI engines is computer controlled injection. Specifically to get the DI engines quieter they inject a small amount of fuel then as piston goes down they inject the remainder of the fuel. This is why the new Cummins and other DI diesels are quite.
As far as compression ratio goes, I was wrong. The DI are usually lower but have larger turbos to make up the difference. I have been interested in ways to increase power and MPG of a OM617, so far the best thing I have found is to get a CDI. Actually one thing I have noticed on the OM617 is that the engine heats up quickly, which indicates that it is not thermally efficient. Regards, Trampas -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Peter Frederick Sent: Sunday, October 23, 2005 4:47 PM To: Mercedes mailing list Subject: Re: [MBZ] prechambers, was speaking of loop type GP's There are lots of design compromises in diesel engines, and there are notable differences in DI and IDI engines as currently produced beyond injection type -- notably, the US made DI engines are almost exclusively long stroke, limited rpm engines (3000 rpm max) with low compression and VERY high boost (20 psi) so that the effective compression ratio under load is similar to a Benz or Volvo short stroke, high rpm engine (up to 5500 rather than 3000 rpm), high compression rato (21:1 and up -- Volvo diesels may be as high as 26:1 as built). However, the direct injection engine WILL produce more hp and torque at the design rpm due to better combustion efficency (sideways sprays out from the injector rather than a directed flame from a pre-chamber) -- but ONLY at that rpm, not much higher or lower. They toss rods when oversped significantly, like any other engine. The cost is NOISE, and since most US diesels don't have variable injection timing, smoke and poor efficiency off peak rpm. Used to be much worse, just watch an old Detroit, Mack, or Cummins over the road truck at a stoplight -- about the time the black smoke starts to taper off under load, the driver has to change gears! And I'm not talking just a little smoke, either! In modern engines, especially the Cummins sixes, the smoke problem is mostly gone unless "user modified", but the noise and inflexibility are still there, hence the lousy milage those engines produce. 17 mpg in a large empty pickup at 65 mph with a diesel is horrible -- my 72 280 SE 4.5 does that good! That engine should produce 25-28 mpg at that speed and load. They are nearly bullet proof, produce prodigous horsepower at peak rpm, and do fine on fuel at that speed. Prechamber engines, on the other hand, are usually very quiet, very flexible (wide rpm range with usable hp and torque), but tend to use more fuel because the thermal effeciency is lower - it's very hard to get excessive exhaust gas temps because the "fire" will be quenched due to lack of oxygen in the prechamber when more fuel is added. On a DI engine, it's fairly easy to keep adding fuel and air and melt the pistons becuase all the fuel will burn every time. The lower thermal effeciency is partially overcome by the higher compression ratio, since higher compression will give you better themal usage reguardless of other design considerations (and they are easier to start cold, too!) The fuel efficency overall, however, isn't that simple. Once you're off the peak rpm in a DI engine, fuel use goes up fast and hp/torque goes down fast, so unless you have an infinitely variable transmission or LOTS of gear ratios to choose from, the speed at which you get good milage is restricted. This was a major problem when speed limits were lowered in teh 70s as most trucks were running at the wrong rpm -- in those days, the power band was as small as 1900-2200 rpm! Off that, no power, stalling on hills, etc, and running a gear down results in even worse milage.... It's great fun to read engine specs for diesels of all types -- the difference between US practice and everyone else in the world is astonishing. Both Detroit and Volvo make large marine engines (3000 hp or so) -- the Detroit is a huge, triple turbo 18:1 engine that weighs almost twice as much as the Volvo Penta -- 23:1 compression, 5000 rpm vs 2200, since tubo, considerably smaller. Both have a history of dependability, but I'mm willing to bet the Volvo uses somewhat less fuel in varied use.... The new Jeep diesel is a Detroit, typical low compression engine, and while it has good torque numbers, the milage STINKS -- barely 20! My brother's SDL gets 28 on the highway, hauling a heavier body with only a four speed, not a six speed tranny. I suspect a 60x turbo Benz 2.5L would give more power, with less fuel consumption (and not much more in cost, either, since that Detroit adds $4k to the price!) There are lots of ways around the barn with internal combustion engines, and diesel engines haven't had the physics worked out as well as gasoline engines yet (ie they weren't used in aircraft, so real effeciency wasn't an issue!). Benz has always landed on the side of extreme longevity and city drivability for the simple reason that the vast majority of Benz diesels were (and probably are today) used in taxies. Over-the-road trucks in US form need the same longevity, but fuel consumption was never a real issue except for a brief period in the late 70s and today, as the cost is easily passed on to the consumer with no direct connection. A taxi service will founder quickly if fuel consumption is high, as most places regulate taxi fares! One thing I expect to see happen to diesels is the required use of biofuels, not so much for fuel reasons as for emissions reasons. Biofuels, since they are oxygenated and don't contain aromatic hydrocarbons, produce both less soot and "cleaner" soot with less aromatic content. The carbon dust isn't nearly so much a problem as the benzene, toluene, and related compounds in it, and nearly everyone who has tried biofuels remarks on the reduction in visible soot production. Peter _______________________________________ For new parts see official list sponsor: http://www.buymbparts.com/ For used parts email [EMAIL PROTECTED] To Unsubscribe or change delivery options go to: http://striplin.net/mailman/listinfo/mercedes_striplin.net
