How The US Military’s ‘Multifuel’ Truck Engines Ran On Diesel and Gasoline

Diesel is a great fuel if you’ve got a diesel engine. Put gasoline in, though, and you’ll risk wrecking it to some greater or lesser degree. That is, unless, you had a magical “multifuel” engine that could run on both fuels. Well, it’s not magical, because — on some vehicles like the legendary “Deuce And A Half” — you could run all sorts of various fuels on the same motor.

On the surface, this sounds too difficult to be practical for traditional reciprocating internal combustion engines. Diesel and gasoline engines run very differently. A diesel engine uses the heat of incredibly high compression to ignite a very unexcitable fuel. Put gas in and it might stumble and choke, or the fuel might detonate under intense compression and damage the engine. Meanwhile, gasoline engines use electrical sparks to ignite an incredibly volatile fuel. Put diesel in, which is hardly volatile at all, and you’ll get crappy, smoky combustion, if the engine runs at all.

Creating a single engine that will run on both of these fuels is obviously a serious challenge. And yet, when NATO military planners in the mid-20th century decided this was important, the engineers got it done. The result was a family of multifuel engines that ran on everything from gasoline to diesel, jet fuel to kerosene, and some other vaguely flammable junk besides. Let’s explore how!

Keep ‘Em Rolling

The appeal of a multifuel engine is obvious from a military perspective. In the middle of the Cold War, planners figured they could end up fighting anywhere in Europe, with nukes blasting big holes in production infrastructure and logistics chains alike. It would be great if your vehicles could run on whatever fuel was on hand on a raging battlefield.

To that end, a variety of NATO countries began exploring multifuel technology. In particular, the US military wanted this kind of flexibility for its truck fleet. It ended up with a variety of multifuel engines from Continental.

[Ed Note: Continental isn’t the same German auto parts/tire company you’re probably thinking about. We’re talking about Continental Motors, which was based out of Michigan and had a major manufacturing plant in Detroit, just overlooking what is now the Jefferson North Assembly Plant (where the Jeep Grand Cherokee has been built since 1992). I used to explore this old plant, and it was simply incredible:

You can see “CONTINENTAL” on that smoke stack. -DT].

They powered the M35 2 1/2-ton truck (called the “Deuce and a Half”), as well as the M39 and M54 5-ton cargo trucks to boot. These trucks also ran a number of other gasoline and diesel powerplants, but it was the Continental engines that gave them the unique multifuel capability.

The first production engine in this line was the Continental LDS-427-2. The designation was an acronym—L stood for liquid cooled, and S stood for turbosupercharger—a turbocharger in modern parlance. But it’s that D in the middle that was key. D stood for diesel, but it doesn’t really refer to the fuel directly. Instead, it stood for the fact this was a Diesel cycle engine—one that runs on compression ignition. This multifuel engine could run on a wide variety of fuels, but no matter which, it used compression ignition.

Continental would go on to build the newer and larger LD 465-1, which had no supercharger or turbo, and the LDS 465-1 and LDS 465-1A, along with a variety of other sub-models. They all used the same basic technology to achieve multifuel operation.

How It Works

At this point, it’s worth talking about difference between gasoline engines and diesel engines. Traditionally, a gasoline engine—whether carbureted or fuel injected—would suck in a mixture of fuel and air on the intake stroke, and compress that mixture. This limits the maximum compression ratio, as if it goes too high, the fuel-air mix will undergo pre-ignition. This usually leads to a high temperature and pressure spike that damages or destroys the engine.

In contrast, diesel engines time the injection of fuel right before the power stroke begins—at the instant it’s ready to combust. On their compression stroke, the engine is only compressing air, so there’s no risk of pre-ignition. Then, when the fuel is injected into the hot, high-pressure environment of the cylinder, it quickly ignites and burns at a stable rate, beginning the power stroke.

For an engine operating on compression ignition, you want fuel that auto-ignites readily. The fuel should have a short ignition delay—basically, that means the fuel ignites quickly once it is injected into the hot, high-pressure environment on the engine’s compression stroke. Good diesel has a short ignition delay so that it auto-ignites as quickly as possible after injection. It also has a lower autoignition temperature than gasoline.

This quality is measured with something called cetane number; basically, the higher the cetane number, the shorter the ignition delay. Diesel typically has a cetane number from 40 and up in the US; the EU demands higher quality diesel with a minimum cetane number of at least 51. In contrast, gasoline often has quite a high ignition delay (and thus low cetane number)—because auto-ignition is undesirable in spark ignition engines. In spark ignition engines, you don’t want the fuel to light itself off, you want it to wait for the spark.

There are a great many fuels that work well in a compression ignition context. Diesel being the foremost among them, but you can use all kinds of other hydrocarbon products. Indeed, some people find great success in running their diesel vehicle on used cooking oil, which has similarly suitable properties for use in a compression ignition engine. Biodiesels produced from animal fats or vegetable oils are also viable, as are a wide variety of kerosene-based fuels and marine fuel oils. These all have good lubricity for keeping fuel pumps and the rest of the fuel system happy, and they have cetane numbers in the acceptable range for good performance.

In the case of the Continental multifuel engines, the military had a simple guide as to which fuels the engine could acceptably run on. Actual diesel was the most desirable choice, along with MIL-F-16884 marine fuel oil and CITE MIL-F-46005 compression ignition fuel. If none of those were available, the engines could run on various “second-choice” fuels—these were kerosene-based jet fuels, like Jet A, Jet A-1, or NATA F-34 or F-35.

The fuel of last resort was MIL-G-3056 combat gasoline, though one can imagine in a difficult enough situation, any gasoline might be worth a shot. However, you’d want to stick to lower-grade gasoline on Continental’s recommendation. Higher-octane grades tend to have higher ignition delay that makes them even worse for compression ignition use.

Normally, you’d expect gasoline would make a diesel engine run poorly, if at all. The injection of fuel in a diesel engine is delicately timed to happen at just the right point in the compression stroke, such that ignition occurs at the right time to create peak power to push the piston back down. With the longer ignition delay of gasoline, it would take too long to ignite to create pressure at the right time on the power stroke to thrust the piston back down. Or, the high heat and pressure might just make the volatile gasoline detonate uncontrollably, damaging the engine with a pressure spike rather than burning in an even, controlled fashion.

There’s also the problem of the fuel pump and injection machinery. Diesel fuel pumps are designed with very tight tolerances and operate at extremely high pressures, with huge forces involved. They rely on the properties of the diesel fuel for lubrication and their correct operation. Running thinner gasoline through the pump, which has no lubricating properties to speak of, will typically destroy the pump in short order. However, unique design choices enabled Continental’s multifuel engine to get around these problems.

Continental referred to its engines as running a four-stroke compression ignition cycle, which it termed “hypercycle” in its internal documents. I’ll explain the cycle first, then we’ll cover how it enables multifuel operation.

On the intake stroke, the intake valve opens as the piston travels down in the cylinder, with atmospheric pressure or the turbocharger forcing air into the cylinder. The intake manifold and valve ports were designed to create a swirling effect as the air travelled into the cylinder and the combustion chamber on top of the piston. As the upward compression stroke begins, the air swirl continues, with compressed air in the combustion chamber reaching 900 to 1000 °F. So far, so normal. As the piston gets near the top of the compression stroke, 27 degrees before top dead center, the fuel injector squirts fuel into the cylinder. This is where things get interesting.

As the fuel is injected, around 5 percent quickly becomes atomized into the air space in the combustion chamber, ignites, and serves as a “spark plug” for the rest of the fuel. At the same time, during the ignition delay period, the other 95% of the fuel ends up in the combustion chamber in liquid form and is exposed to high temperatures, but remains below the fuel’s “cracking temperature.”

Basically, the fuel gets hot enough to start vaporizing, but the hydrocarbon chains of the fuel don’t actually start breaking down. This bulk of the fuel charge starts vaporizing due to the heat and is swept around the combustion chamber by the still-swirling air, burning smoothly as the power stroke sees the piston heading downwards once again. The air swirling in the chamber continues to gradually pick up the vaporizing component of the fuel and helps it combust evenly during the power stroke, creating “even combustion and eliminating detonation knock.” Finally, the power stroke finishes, the exhaust valves open and the upward exhaust stroke begins to purge the combustion products from the cylinder.

Basically, the difference in the hypercycle mode of operation is all about the combustion chamber and the manner of fuel injection. See, in a diesel engine, you’d traditionally want to inject an atomized stream of fuel that would combust relatively quickly upon injection. However, if you did that with gasoline, it would detonate in a nasty fashion since the atomized fuel would quickly vaporize and then combust all at once. Instead, by injecting a largely-liquid stream into the deep spherical combustion chamber cup on the piston, the fuel only gradually vaporizes and burns in a more controlled manner. This slow burn avoids detonation and knocking from rapid combustion events, to the point where even a fast-burning fuel like gasoline could be realistically used in such an engine.

The technology was actually first developed by German diesel manufacturer MAN, and picked up by Continental years later and termed “Hypercycle.” The idea of injecting fuel into a deep combustion chamber on the piston was called M-System at MAN, and was used on a range of diesel engines. At first, it was just a decent way of mixing fuel and air in a compression ignition engine. The fact that this technique lead to a gradual vaporizing of fuel made it suitable for multifuel use which came about sometime later. The concept is discussed in a patent, #2,907,308, which took me all day to find. Filed in 1955, it explains the technique of using the deep spherical combustion chamber to create slow, complete combustion of fuel.

Practical Concerns

In general, the trucks and the engines that ran them were designed to run on whatever suitable fuel was put in the tank, with no modifications required. However, in reality, it was often desirable to take some consideration in which fuel you ran in the engine, and how. For example, you could run on straight gasoline if you really had to. You’d probably find the engine would run better if you cut that gas with diesel where possible. This was particularly relevant for premium gasoline, if you happened to come across it. Adding some diesel would slash the octane rating and speed up the ignition delay, making it a little more suitable for compression ignition.

Funnily enough, these engines did have spark plugs, too, but not for the reason you’d think. They were not used for ignition of the fuel-air charge in the cylinder. Instead, they were used as part of the “flame-type” manifold heater. Basically, instead of glow plugs, the engine would instead use a flame heater to warm the intake air for cold starting. This was achieved with a nozzle that sprayed fuel under pressure into an elbow hanging off the side of the  intake manifold. A spark plug in the elbow ignited this small amount of fuel, burning it in order to warm the air in the intake.

It was crude, but effective. These days, engines tend to rely on electrical heaters in the intake instead, which offer lower complexity, but some problems of their own.

Given these were mechanically-controlled engine from the mid-20th century, it was fairly crude in terms of its state of tune. The engine could run on a variety of fuels, but it sacrificed peak performance for flexibility. The LDS-427, for example, had a displacement of 427 cubic inches, or 7.0 liters. It managed to produce 130 net horsepower running on VV-F-800 diesel, or a lesser 118 hp on compression ignition fuel compliant with MIL-F-45121. It would achieve just 103 horsepower on gasoline by comparison. Similarly, diesel would net you a mighty 330 pound-feet of torque, but that would drop as low as 280 pound-feet on gasoline.

The later LDS-465 was a larger displacement engine, measuring 478 cubic inches or 7.8 liters. In turn, it had higher output of 170 to 185 horsepower, and a maximum torque output of 440 pound-feet of torque. It had the bonus feature of a “fuel density compensator.” This component would help meter different amounts of fuel to the engine based on the density of the fuel, since there are differences between the densities of diesel, turbine fuels, and gasoline.

Decline

Multifuel engines promised to keep American logistics rolling in difficult battlefield situations. The same logic was applied to a variety of other NATO vehicles, including a number of main battle tanks. Some relied on similar reciprocating internal combustion engines, others used gas turbine designs that were inherently less fussy about which hot liquid was burning inside them.

Ultimately, though, the concept has fallen by the wayside. These days, the vast majority of NATO combat vehicles all run on one fuel—JP-8. This was introduced in the 1980s as the “Single Fuel Concept.” JP-8 is close enough to diesel that it works pretty much fine in diesel engines, and it’s perfectly suitable as jet fuel, too. NATO militaries have focused on fielding vehicles that run on this single fuel and largely forgotten about the hassle of working with gasoline entirely.

In any case, these engines are a great example of what can be achieved in the world of internal combustion. Sure, these engines weren’t super potent on diesel or gasoline, and they weren’t winning any competitions in the efficiency stakes. But you could fill them up with just about anything flammable, anywhere in the world, and they’d get down the road regardless. That’s an impressive feat, whichever way you look at it.

Image credits: Sauerlaender, CC BY-SA 3.0, Continental, US military (public domain), Alf van Beem (public domain), Zephyris, CC BY-SA 3.0