I spied this up at busted knuckles described as a two stroke Diesel engine. And so it is. Now - I am not a gear head. I know there are two stroke diesels but I don’t understand their purpose. Traditionally, I understand two strokes as being the preferred choice where a high power to weight ratio is needed. Say… a chainsaw or a weed whacker. That’s the way it is for gas engines, for the most part, anyway… so: what is the place or niche for the two stroke diesel? I see the classical light engine… but a heavy gear pump or blower bolted on it…so there goes your power to weight ratio, doesn’t it?
I have been able to clear up another misunderstanding: it used to be that when I thought about Diesel engines, I automatically assumed they burned diesel fuel. Cederq has since corrected that for us; diesels achieve their power stroke by heat of compression - and so they can use any fuel that can burn under those conditions. They can be normally aspirated too. It used to be that small Diesel engines were used in RC Airplanes that burned a mixture of ether, castor oil and paraffin.
Those gave way to the engines I use today. Mine are glow engines that burn nitromethane. We call them glow engines and I wonder if they aren’t technically diesels too? They use a glow plug to initially fire the engine and then maintain heat for the power stroke at lower compression…I think. Like diesels…they come in 2 and four stroke versions too.
Why would you want a two cycle Diesel engine with a blower on it?
Two-strokes are simple to machine. A blower helps with scavenging and increases thermo-mechanical efficiency.ReplyDelete
We have come a long way since the 1930s when the idea of using the gas's velocity and momentum to "tune" the engine and optimize extraction of the exhaust gasses and "packing" of fresh intake was inconceivable.
Some of those early two-strokes had a single, exhaust valve in the head so the spent gasses moved from intake port to exhaust in a straight-ish line.
Lot more power in the old 8-92 Detroit. Like driving a giant chainsaw. It was quick.ReplyDelete
When I was in the Army, we had 2-stroke Detroit Diesel engines in armored personnel carriers (6V53T) and self propelled field artillery (8V72T). The had an exhaust driven turbo charger for power, and a mechanically driven super charger to blow fresh air through ports into the cylinder while blowing exhaust gas out the exhaust valves. If the quill shaft which drove the super charger broke, the engine would continue to run at very low power due to the effect of the turbo charger, but if shut down, it would not re-start. So bottom line: that 2-stroke engine would not work without a super charger.ReplyDelete
Two cycles have more power per cylinder because they are powered on every downstroke, instead of every other downstroke. All piston engines are powered by an explosion in the combustion chamber, and increasing the force of the explosion by increasing the ammount of explosive will also increase their power. A blower can blow more fuel/air mixture into the piston at higher pressure. These are usually called superchargers (when they are powered by the engine), or turbochargers (when they are powered by exhaust pressure). The greater forces require stronger (heavier) engine parts, so the effect is only scalable to a point.ReplyDelete
As ERJ points out, the real beauty of the 2cyl is that they can be made very simple and with fewer moving parts... at the expense of reduced effeciency. If your turdbird has a glow plug, but no injector, then ignition timing is thrown to "the wind" of engine speed, temperature, and fuel properties.... but it don't burn that much fuel, so who cares about effeciency?
Some 2cyl engines (those with no cylinder ports only and no "valves") are very sensitive to exhaust backpressure: too much pressure and you don't exhaust completely, and get less fuel/air in the chamber.... too little and you throw fuel/air out the exhaust unburned. For these engines you can make a passive turbocharger called an "expansion chamber", which is a big bubble in the exhaust pipe at the exhaust port. this makes the back pressure low in the beginning of the exhaust cycle, so excess fuel/air goes out into the exhaust chamber... then "bounces" (resonantly) off the chamber, pushing that excess *back* into the chamber after the piston has cut off the intake port. This makes an engine much more powerful, but only in a narrow RPM range (resonant with the chamber size)... hence your "wing ding" sound of racing 2cyl bikes as they run like shit at low RPMs missing more than they are firing.
Detroit Diesels were 2 stroke, great marine engines, lighter than the same power out for Cat &etc Downside was higher fuel usage, and not being able to meet newer enviro regs.ReplyDelete
Simple to maintain too, fewer parts, quite reliable.
The old 2 stroke Detroits were uniquely efficient at turning diesel into decibels.ReplyDelete
Many of the older locomotives used the 2 stroke diesels with a turbocharger. The new emissions standards have forced the manufacturers to go to 4 stroke diesels.ReplyDelete
The Amish here covet the 2 stroke diesel for their dairy parlors. They use them for many other things as well. I have an an 86 military Blazer with a Detroit in it. It accelerates like a snail, but it will pull stumps all day, and I haven’t got stuck in deep snow or mud yet...ReplyDelete
I'll admit I'm only familiar with the Detroit Diesel family of 2-stroke Diesels, so I can't speak to any other designs, but as was pointed out in an earlier comment, the DD 2-cycle requires the blower to provide positive airbox pressure (approx 2" - 4" H20, if I recall correctly) to fill the cylinder with fresh air and push the exhaust out (scavenge).ReplyDelete