What is the reason for having the two separate kinds of engine? Is it impossible to design a diesel cycle engine that uses petrol, or an Otto cycle engine that uses diesel? It seems like it should be well within the realms of possibility, so it must be an efficiency/performance argument. In that case, what is it about the nature of the two fuels that make them better suited for one or the other cycle?
The Diesel engine is more efficient than the Otto cycle engine. It also uses heavier, energetic fuel that requires less refining and is less flammable. In fact, the diesel engine can run on a variety of fuels. On the downside, it has higher compression and thus more significant engineering, and fuel injection systems are more complex than carburetors. They are also harder to start, and the fuel can gel in the cold.
Petrol engines run at lower compression, and have (at least initially) a carburetor which is a fairly simple system. They are easy to start, and the fuel remains good in the cold, but does not keep well for extended periods of time. However, the fuel is harder to refine and needs igniting at point of maximum compression, introducing an electrical ignition system.
A diesel cycle engine cannot use petrol because of the burn characteristics - if you compress it enough to self-ignite, it can detonate (pre-ignition) ahead of the appropriate point of the cycle. This will damage the engine. An Otto cycle engine cannot use diesel because it will not compress enough to ignite cleanly (if at all), the plugs will foul and the engine will choke.
Different fuel characteristics, different engines, different uses. However, they do overlap in some spaces - I love my Passat TDi, perfect for a long daily motorway commute. Efficient, cheaper than petrol, and reliable. However, YMMV (literally, depending on how you drive). 
Si
You’ve made reference to the idealized thermodynamic Otto and Diesel cycles, so I’ll start by pointing out something significant that most folks don’t realize. There are three things that make a real-world Diesel engine more efficient than a real-world spark-ignited (SI) engine:
-unthrottled intake. The Diesel engine isn’t expending energy to suck air past a partially closed throttle plate. This is most noticeable at part-load operation; diesels deliver good efficiency even at light loads, whereas SI engine efficiency falls off rapidly as you go to lighter and lighter loads.
-high compression ratio. The upper bound on an engine’s thermodynamic efficiency is set by the compression ratio. Because SI engines depend on the fuel remaining unignited until the spark is delivered, they are limited in how high a compression ratio they can use. The best SI engines in production today can operate at CR’s as high as 14:1; Diesels are closer to 20:1.
-lean operation. A Diesel engine always sucks in more air than it needs to burn the fuel that gets injected. The unused excess air acts as a diluent, lowering the final temperature of the burned mixture, which reduces heat loss to the combustion chamber walls, leaving more heat available to be converted to mechanical work during the expansion stroke.
OK, so here’s the thing most folks don’t realize: *those three things are not defining features of the idealized Diesel thermodynamic cycle. * In fact, the only difference between the idealized Otto Diesel cycles is the nature of heat addition. The idealized Otto cycle uses “constant-volume heat addition,” i.e. the heat is all added instantaneously at TDC to approximate rapid premixed combustion, and the idealized Diesel cycle uses “constant-pressure heat addition,” i.e. the heat is added gradually during the early portion of the expansion stroke to approximate the slower diffusion-flame combustion of a diesel engine.
Anyway, that’s enough theory. As for reality, why can’t we combine the best features of each type of engine? A gasoline SI engine has low emissions, and a diesel engine has high efficiency. Can we put the two together?
It turns out we can, but the resulting beast is horribly fussy and doesn’t lend itself well to direct operator control. The technology is called Homogenous-Charge Compression Ignition, or HCCI, and it works like this:
-gasoline is injected into the intake ports (just as in an ordinary SI engine), where it mixes with incoming air; the mixture then enters the cylinders.
-The mixture is compressed to a degree typical of diesel engines, around 19 or 20 to 1. At some point late in the compression stroke, the mixture ignites, simply by virtue of the high temperatures produced during the compression process (same as an ordinary diesel engine). Because the fuel is already completely vaporized and the fuel and air are premixed just as in a traditional SI engine, combustion is much cleaner than it is in a diesel engine, where there are huge local variations in temperature that result in high production of soot and NOx.
Hey, great, we’ve got an engine that operates at high compression ratio, with a lean mixture, and no intake throttling. We can get efficiencies typical of conventional diesel engines, but because the fuel/air is premixed, it burns nice and clean. Winning!
Sounds simple. However, the timing of that ignition event is critical to engine efficiency, and in an HCCI engine it’s both extremely sensitive to initial conditions (air-fuel ratio, EGR rate, intake air temperature, etc.) and extremely unstable. “Unstable” means that once the ignition timing starts to deviate (over the course of several combustion cycles) from a particular timing, it tends to get worse and worse rather than self-correcting. If ignition starts a bit earlier on one cycle, then combustion temperatures get hotter, which heats cylinder walls more, which leads to earlier ignition on the next cycle, which leads to hotter walls, on-and-on, until combustion is happening way too soon and possibly even transitioning to detonation. If ignition starts a bit later on one cycle, you’ll see the opposite progression until ignition stops happening altogether. HCCI also works only within a relatively narrow band of RPM’s and power outputs, which means it’s not suitable for use in a conventional drivetrain where the driver issues commands through an accelerator pedal and expects prompt and wide-ranging response from the engine. Instead, HCCI (so far) is only suitable for use in series-hybrid drivetrains, where the engine is completely under the control of a computer. The computer is able to analyze the pressure trace from the most recent combustion cycle and determine - prior to the next cycle - whether to add more/less fuel, more/less EGR, or turn on intake heaters as needed to keep the ignition timing where it needs to be. The computer can also limit the rate-of-change of power output and/or RPM to avoid potential instabilities associated with rapidly changing operating conditions.
When you get it to work, it’s pretty cool. Low emissions, high efficiency, the dream combo. The most recent innovation is to create a gasoline mixture that’s leaner than ideal, which prevents it from easily auto-igniting even at high compression ratios - and then apply a tiny spritz of diesel fuel injected into the combustion chamber to trigger ignition exactly at the desired time. That spritz of diesel fuel lights off very easily, and then that in turn starts ignition in the rest of the gasoline-air mixture. Presto: ignition becomes much easier to control. Because only a tiny amount of diesel fuel is injected, it doesn’t produce the awful quantities of soot and NOx that conventional diesel engines do; nearly all of what burns is the premixed gasoline, which as described earlier burns relatively cleanly.
Machine Elf - ooooh, that sounds very cool (well, hot, for precisely defined values of hot, to control ignition). Combustion by computer.
Si
Machine Elf: Thanks for the great explanation. It’s posts like yours that keep me coming back here.
Quick question: Are model airplane engines such as the (now-defunct) Cox .049 two-stroke Diesel engines?
They run on a mixture of Methanol and Nitro-methane (plus lubricating oil). They self-ignite due to compression, so are similar in that regard. They do require the glow plug to be heated up, though.
There were other model aircraft engines that were diesel engines, and there are conversions for Cox engines to diesel fuel (new head and heavy duty crank. I don’t think they require glow-plugs.
Si
They are glowplug engines. They sort of rely on compression, but they won’t run without a hot, catalytic glowplug that’s included with the replaceable head. To start these, you had to connect a battery that powered a heating element in the glowplug. Once the glowplug was hot, you could get the engine started with a deft flick of the propeller; once it was running, you could remove the battery, and normal operation kept the glowplug hot enough to light off the mixture somewhere close to TDC. There was a tiny bit of platinum catalyst on the glowplug too, and it was critical. I’ve seen engines like this where, for whatever reason, the platinum had gone away, or somehow been poisoned or sintered. With the battery hooked up, the glowplug was hot enough to keep the engine running even without that catalytic effect - but when you removed the battery, as soon as the glowplug cooled down just a tiny bit, the engine would die.
The nitromethane is critical in these engines. They run at low compression ratios with minimal air intake. Only a fuel like nitromethane containing a large proportion of oxygen would work in this simplistic engine.
Ah, I see. From Wiki:
The Cox .049 was fun, though. Mine had lost the spring on the shaft to spin the prop, so it was finger-flicks (and a few bruises, too). If it started the wrong way you dropped a rag into the prop and tried again. The noise and the smell of castor oil/nitromethane were great.
It was also so much fun taking the thing apart to see how the engine worked. Takes me back. Of course, I never was very successful at flying the plane it came with (Stringline control), but still. I learned a lot from that engine.
Si
Those engines were really cool. I had planes and cars that used them. Starting them and keeping them going was a major hurdle. I had a plane on a line like that, and it ended up in a spectacular crash into a parking lot after entering some kind of oscillation that I couldn’t bring it out of. A friend of mine had an elaborate model plane (old fashioned balsa and paper), but he could never keep his engine running long enough to get it in the air. Definitely good times. Don’t know how kids will learn anything anymore without that kind of hands on experience.
Don’t diesel engines produce a lot more soot (particulates) than gasoline engines ?
So called “Diesel” model airplane engines are true compression ignition, but they lack the injection system of true Diesels. They pre-mix the air and fuel in a carburetor just like a glow-plug engine. In addition to fuel oil and lubricating oil, the fuel mixture contains a lot of ether. The ether allows ignition at fairly modest compression ratios, and also gives a much more gentle ignition event than fuel oil alone would have.
Compression ratio is varied to control ignition timing, though mixture can be adjusted and also has a significant effect. The cylinder head contains a second stationary piston (counter-piston) that is positioned by a compression adjustment screw. The Cox .049 conversions (Made by Davis Diesel Development) used a teflon diaphragm under the counter-piston, so the fit between the head and counter-piston was not critical. This also provided some insulation improving ignition.
The two interacting adjustments of mixture and compression require a bit more experience to successfully start, and the ether makes diesel fuel stinky and a bit more hazardous than glow fuel, (which is not all that safe anyway) As a result, these Diesels were never very popular in the US. They were fairly popular in the UK and Europe, due to a scarcity of the Nitro-methane needed to get decent power out of glow-plug engines. Glow engines will run fine on straight methanol and lubricating oil, but Diesels have a much better power to weight ratio.
I knew this bit, which is why…
… I expected this could be done, and wanted to know…
What you told me here! Thanks!
It was the almost-finger-off experiences that provided the real learning. :smack:
Si
Side note: When I was in the motorcycle biz, I had the mis-fortune to work with a group of guys who were converting single-cylinder 2-stroke engines (Honda CR500’s, to be specific) to run on diesel for the military.
They were World-Class pains in the ass!
There are engines used in military trucks and tanks that can use both gas and diesel (as well as kerosene, jet fuel, and anything else vaguely resembling a liquid hydrocarbon), but there is a major performance loss when running anything other than the primary fuel (usually diesel) and even then performance is not what it would be in a single-fuel engine.
I’ve heard diesel fuel has a flash point higher than room temp, so it won’t ignite normally, is this true?
That may be sort of. My understanding and/or this description may be lacking. It appears that unless you atomize kerosene at room temperature, there isn’t enough surface area to burn and generate enough heat to maintain a flame. But the conditions in a diesel engine cylinder are different. Under high pressure the flash point is exceeded. The diesel engine ignites the fuel by increasing pressure until the auto-igniton conditions of the fuel are exceeded. The progress of the combustion in a cylinder is more complicated than that, but the difficulty in igniting kerosene is what makes the combustion controllable since it’s occuring throughout the fuel much faster than the reaction in a gasoline engine.