Today’s world of heavy-duty diesel engines is almost entirely dominated by large inline-six four-stroke engines, but that wasn’t always the case.
As a matter of fact, there was once a point in time not too long ago when two-stroke diesel engines dominated the market.
So today, we’re going to rewind the clock and take a look at Detroit Diesel’s infamous two-stroke diesel engines and what makes them so good.
Where the Two-Stroke Diesel Came From
To understand where the two-stroke diesel engine came from, we first need to rewind the clock and understand where Detroit Diesel came from in the first place.
Back in the late 1920s and early 1930s, general motors were in a good spot to buy new companies, as the stock market crash of 1929 massively devalued many companies in the automotive space.
GM looked into a handful of different companies with plans to purchase a company that could produce top-notch diesel engines.
They even had the opportunity to purchase Cummins at this time, but ultimately passed on them to purchase a company known as Winton, as well as Electro-Motive Company.
It’s crazy to think how much different things could’ve been if GM had purchased Cummins all the way back then, but as they say, hindsight is 20/20.
Between GM, Winton, Electro-Motive, and a smart inventor named Charles Kettering, they took existing Winton two-stroke diesel engines used for Yachts and improved upon them massively, mostly working on perfectly the unit injector.
By 1938, Winton became the Cleveland Engine division under GM, with the goal of continuing to produce and sell marine and stationary power applications.
And one year after forming that new engine division, they decided to take all the R&D that they had already done on these large two-stroke diesel engines and shrink it down to a smaller platform, one that wasn’t meant for marine, locomotive, or stationary use.
And to do this, they created an entirely new engine division known as Detroit Diesel.
The Series 71
That takes us to Detroit Diesel’s first engine, the Series 71.
And although the Series 71 was based on the research and development of the Winton two-stroke diesel engines, the Series 71 wasn’t just a shrunken version of those engines.
Rather, it was an entirely new engine platform that was completely unrelated to those earlier two-stroke diesels.
With the all-new Series 71 engine platform, there were three sizes available right off the bat.
You could either get a 3-cylinder, a 4-cylinder, or a 6-cylinder.
By 1938, Detroit Diesel ramped up production of their new engine, and GM sent around 700 engines to their Truck and Couch division, and then in 1939, they sent the new Series 71 to various suppliers for use in their applications.
Fast forwarding a little bit, the Series 71 was used all across World War II, and by 1944 a whopping 62,000 of these engines were produced.
After the war, though, things started to shift in America.
Where transporting goods across the country with rail cars was once the norm, suddenly, over-the-road trucks started to rise in popularity as a viable alternative to rail cars.
And this was really made possible thanks to the Federal-Aid Highway Act of 1956, which had a huge impact on in-city travel as well as inter-state travel.
By then, Detroit had also introduced two other engine series: the Series 110 and the Series 53, both of which were also two-stroke diesel engines.
Okay, that’s enough about how the two-stroke Detroit Diesel came to life. It’s time we take a deep dive and look at how this actually works.
How the Two-Stroke Diesel Works
Now rather obviously, a two-stroke diesel engine operates quite a bit differently than a conventional four-stroke diesel engine.
With a four-stroke diesel engine, there is the intake stroke, compression stroke, ignition stroke, and exhaust stroke.
With a two-stroke diesel, though, intake, compression, ignition, and exhaust all happen in two strokes and one rotation of the crankshaft, which, put simply, means for every one crankshaft rotation, there is a full engine cycle rather than half an engine cycle.
In this diagram here, you can see how this works, with the intake port of the cylinder located at the bottom of the cylinder on the wall rather than the intake valve at the top of the engine, but this presents a big problem already.
With the intake stage happening at the bottom of the stroke, there is no vacuum to pull the air into the cylinder like a four-stroke engine does, which means two-stroke diesels require some sort of forced induction just to operate normally.
And while there is no intake valve on the cylinder head, there is still an exhaust valve up there, as well as the fuel injector.
The way this works is by effectively having the intake and exhaust stage happen at the same time.
As the piston reaches the bottom dead center after the combustion cycle, the exhaust valve opens around the same time the intake port is revealed, and the air being pushed into the cylinder by forced induction also helps to push out the exhaust.
What Are the Benefits?
So at this point, you have an understanding of where Detroit Diesel came from and how their two-stroke diesel engines function, but now the question is why?
Why bother with all of this? What are the benefits of using a two-stroke diesel instead of a four-stroke diesel?
For one, the two-stroke diesel design is not only simpler with fewer moving parts, but it’s also quite a bit lighter, smaller, and easier to manufacture. Granted, the weight of the engine isn’t particularly important for over-the-truck applications, but it’s still a benefit.
Also, as a side note, lightweight is a very loose term, as even the smallest three-cylinder 71 series engine came in at over 1500lbs.
I think it’s safe to say this isn’t something you’re swapping into your Miata.
With fewer moving parts to fail, there also comes the benefit of reduced maintenance, at least in theory, which is a massive plus, because when your truck breaks, the money stops coming in.
And because the weight is low, the physical size is compact, and there are twice as many combustion events per crankshaft rotation, the power-to-weight or power-to-size ratio of a two-stroke diesel engine is far past that of a four-stroke diesel engine.
And although you could argue this is less of a benefit than it used to be, having the ability to run on different fuels, including heavy fuels and low-quality fuels, is a pretty big advantage, particularly in certain applications where fuel availability or quality is a concern.
And with these advantages, it’s pretty clear to see how the Detroit two-stroke diesel engine became so popular, especially after they introduced the V engine variant of the original 71 series engine.
With this new V layout, Detroit was not only able to offer engines with more power than ever before but also offer a variety of different displacements, giving you more choices than ever. The options for the V71 include the 6V71, 8V71, 12V71, 16V71, and, much later on, the 24V71.
What Are the Drawbacks?
So, if the two-stroke diesel is so good at so many different things with a host of different engine sizes to pick from, where are they now?
Why isn’t this engine still around and being used if it’s so good?
And the answer can be summed up in a single word: emissions.
Two-stroke diesel engines, including those from Detroit Diesel, were faced with some serious challenges in meeting stricter emissions regulations.
Keep in mind, the EPA wasn’t even formed until 1970, so the idea of reducing or limiting emissions of any given engine wasn’t really a thought or concern for diesel engine manufacturers.
With two-stroke diesel engines, the scavenging process can result in higher emissions of pollutants such as particulate matter.
There is also the issue with fuel efficiency at low loads, with two-stroke engines being worse than a comparable four-stroke engine, which simply means these two-stroke engines consume more fuel and output more emissions at idle or low load conditions.
Two-stroke diesel engines also typically have a higher oil consumption rate compared to four-stroke diesel engines, which, again, isn’t great on the emissions front.
As emission standards became more stringent after the creation of the EPA under President Nixon, meeting these requirements became increasingly difficult for Detroit’s two-stroke engines until they eventually just couldn’t meet the on-road emissions restrictions and had to be phased out.
Can the Technology Be Brought Back?
So this brings up the question, with all the pros of the two-stroke diesel engine and the biggest con simply being the emissions output, is it possible to create a two-stroke diesel engine that could pass modern emissions standards?
And the answer is no, sort of.
Detroit did make advancements in the emissions output of their two-stroke engines by adding electronic controls, but eventually, it just wasn’t possible to keep up with the demands.
The underlying issue is that with the two-stroke diesel, the combustion configuration allows unburned fuel to pass out the exhaust port before the port closes, resulting in excessive emissions.
As long as that underlying design is present, it won’t be possible to meet emissions standards.
It’s much of the same reason that smaller two-stroke gas engines are being phased out.
It’s just not possible to meet the EPA’s demands in this regard.
But, that being said, a two-stroke diesel engine is still feasible to use in off-road applications where you can sort of get around emissions testing in some situations, particularly for something like a tractor powered by a Detroit engine.
But for on-road applications, you can’t really get around emissions testing for heavy-duty applications.
Why the Two-Stroke Diesel is So Important
I don’t say this lightly, but Detroit Diesel’s two-stroke engines are undeniably some of the most important engines in America’s history.
With their massive popularity and the shift from rail cars to over-the-road trucking for transporting goods across the country, these engines quite literally helped build America into what it is today.
And what makes them so interesting is that other big diesel companies didn’t even bother competing with their own two-stroke engines.
With companies like Caterpillar and Cummins never made anything but four-stroke engines during the two-stroke’s heyday, putting Detroit in a special category of its own.
At one point, they quite literally controlled around 30% of the market share for over-the-road truck engines and built an absolutely massive amount of these engines, with loads of them still in use today, although the remaining applications are generally not over-the-road trucks.
Interestingly enough, though, someone who did copy Detroit and make their own two-stroke diesel engines is the Soviets, who copied and built them for use in military applications, including a copy of the Detroit 4-71 and the 6-71.
Now as a side note, an industry that still is using two-stroke diesel engines, although not engines built by Detroit Diesel, is marine.
And a really interesting feature for these massive marine engines is that there is no gearbox, which means if they want to turn the prop in reverse, they just shut the engine off and turn it back on, but with the engine spinning in the opposite direction.
That’s something you most definitely cannot do with a four-stroke engine, and it’s a very interesting benefit to the two-stroke engine design.
So, although Detroit Diesel’s two-stroke engines lasted for quite a while and powered countless different applications ranging from military to over-the-road trucks, the introduction of emissions restrictions from the EPA was the beginning of the end for these engines.
Two-stroke diesel engines just can’t meet the emissions standards set for on-road vehicles, and ultimately Detroit Diesel had to eventually switch to only producing four-stroke engines.