Home » The U.S. Military Once Rode 100 MPG Jet Fuel-Powered ‘Combat Motorcycles’ And You Can Buy One Today

The U.S. Military Once Rode 100 MPG Jet Fuel-Powered ‘Combat Motorcycles’ And You Can Buy One Today

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Earlier this week, I wrote about one of the only mass-produced diesel-powered motorcycles ever sold to the public. The Royal Enfield Diesel had the classic looks of a Royal Enfield and an incredibly frugal diesel engine. If you like more military chic, today I have something right up your alley. The Hayes Diversified Technologies M1030M1 is a ridiculously cool off-road combat bike that could hit roughly 100 mpg and be refueled at your local airport.

As you could probably tell by now, I’ve fallen back down the familiar rabbit hole of diesel-powered motorcycles. There aren’t many of them out there, at least compared to all of the gasoline-powered and electric motorcycles that you can find throughout history. The handful of diesel motorcycles that do exist get pretty wild. Sure, you’re not going to find a diesel Hayabusa, though that would be epic, but you will find motorcycles made to just sip on morsels of fuel like the Royal Enfield Diesel from Monday, bikes with diesel car engines, or one meant to help solve the needs of North Atlantic Treaty Organization (NATO) nations.

Vidframe Min Top
Vidframe Min Bottom

That’s what you’re getting with today’s HDT M1030M1. And it’s actually pretty awesome.

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The HDT M1030M1 was birthed out of an effort to streamline fueling in the military. As the Modern War Institute at West Point explains, keeping a wide variety of equipment on the field running required some interesting logistics:

It was 1969, during Capt. Kern’s second tour of Vietnam. His fuel logistics hassles were even worse than those he faced his first time in the country when he was a cavalry platoon leader. During that tour, his soldiers had resorted to hanging bags of fuel from trees so that gravity would refuel their M113 armored personnel carriers. If they were in a good spot, they could drive the vehicles into a ditch and rest the fuel bags on the ground. Now Kern was responsible for not just M113s, but an assortment of forty vehicles including M551 Sheridan armored reconnaissance assault vehicles, a borrowed M548 cargo carrier, occasionally an M48 Patton tank, and M118 trailers. Diesel “recipes” were not standardized at the time and varied based on the season and the region. It took a while for the logisticians to get the local mix right. But the real hassle was the M48 and its gasoline. The gas evaporated so quickly in the hot climate and the engines ran so inefficiently that the beast required refueling twice a day. When soldiers unknowingly supplied a bladder full of jet aviation fuel for the affected vehicles it all had to be pumped out after the fact. Little did Kern know that when he arrived at Ft. Stewart years later as a battalion commander, he’d be dealing with an even greater variety of local diesel recipes, gasoline, and jet fuels for his vehicles.

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As the Modern War Institute and SAE International write, in the late 1980s NATO nations began an effort to streamline the fueling process. Instead of carrying various fuels for different vehicles, the militaries worked towards “one military fuel for all the land-based military aircraft, vehicles and equipment employed on the military arena.” It’s known as the Single Fuel Concept (SFC) and the fuel selected for the task is JP-8 fuel, which is similar to commercial aviation Jet A-1, but contains a corrosion inhibitor, a lubrication additive, and an anti-icing additive. The advantage of JP-8 is that it could be used to fuel a jet and also be used to fuel a truck. Our Jason Torchinsky further notes that this stuff is similar to the fuel used to heat some homes. Or, in this case, JP-8 also fuels a motorcycle.

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The HDT M1030M1 is the work of Hesperia, California-based Hayes Diversified Technologies and the UK’s Cranfield University. Founded in 1961, Hayes had previously worked on other military motorcycles like the M1030, a 1980s and 1990s Kawasaki KR250 adapted for military use. The company also built the M1030B1, a gas-engine Kawasaki KLR650 adapted for military use.

M1030b1 1999 Kawasaki Klr650 Right Side
A M1030B1 – eBay

Hayes, along with Cranfield University, would take the M1030B1 and develop it into a motorcycle that fit the Single Fuel Concept. In November 2004, the EurekAlert! science journal reported that the first diesel-powered KLRs hit the road. It was hailed as the world’s first production diesel military motorbike, and the first motorcycle of any kind to have a diesel engine built for the purpose.

Hayes and Cranfield University had a tall order with this motorcycle. The M1030M1 had to be sufficiently lightweight and powerful enough for tactical use, all while being reliable. And of course, it had to be able to run on diesel or JP-8.

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Engineers from the university John Crocker and Dr. Stuart McGuigan worked with HDT for what sounds like one ridiculously cool machine, from EurekAlert!:

And so powerful is the motorcycle that in September 2004 it set the world’s first land speed record for a diesel fuelled motorcycle.

Fred Hayes, founder of HDT, who was in the saddle at the world famous Bonneville Salt Flats, Utah, said: “The event was marred by rain the previous week and by poor track conditions, which limited the top speeds due to soft, wet salt. The normally aspirated bike was officially timed by the AMA at 85.466mph, against our calculated top speed of 86mph with production gearing. The calculated speed was at sea level (4350ft) on hard pavement. We’re delighted with the result. If we’d had an option for gearing and more track time, we may have broken the 90mph barrier.”

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By the publication of that article, the U.S. Marines placed an order for 522 of the machines with the U.S. Army and a number of NATO nations expressed their own interest. Hayes had beaten an array of other companies for the contract, which even included a team consisting of Harley-Davidson and Lockheed.

The M1030M1 is powered by a 611cc single-cylinder indirect injection diesel making 30 horses and 33 lb-ft torque. That’s down from the 42 horses and a touch under the 34 lb-ft torque of a normal KLR650. HDT advertised that the motorcycle was capable of achieving 96 mpg at 55 mph. The motorcycle also has 60 percent gradeability, an equipment rack that can hold 50 pounds, a water fording depth of two feet, and a 4.2-gallon fuel tank. For those of you counting, that means a range of around 403 miles on a fill.

Hdt
HDT USA

Perhaps my favorite part is that this motorcycle goes against what’s commonly reported about diesel motorcycles. If you search for the downsides of diesel motorcycles, you’ll often find claims that diesel engines are too heavy for practical use in a motorcycle. While this may arguably be true for something like that Royal Enfield Diesel, the HDT M1030M1 was advertised as having a 369-pound dry weight, or just 32 pounds more than the 337-pound dry weight of the donor KLR650.

Reports at the time noted that Hayes was working on a civilian version. However, when Cycle World asked Hayes about it, the company’s founder noted that demand from different militaries was so high that the small outfit didn’t have the workforce or the production line for a civilian version. The military bike did get a sequel with the M1030M2, which bumped displacement up to 670cc and power to 33 HP.

M2
HDT USA

That one reportedly went 121.2 mph at Bonneville.

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While the military ordered 522 of these, some reports suggest that just 440 of them were actually delivered. It’s also unclear how many of the M1030M2 exist. Over the past handful of years, these motorcycles have been coming out of duty and onto government surplus auctions, where they get scooped up by civilians. When I first discovered these motorcycles back in 2018, I saw auctions selling for around $5,000.

Today, I’m now seeing them sell for $9,000, $10,000, or even nearly $15,000 at government auctions. A few readers also tried selling me theirs for $15,000. I couldn’t find any currently up for auction right now, but I did find three for sale in private hands. There’s one on eBay that won’t start for $12,500 and two on Facebook for $14,995 and $20,000, respectively. With prices like these, I fear I missed the boat on a cheap diesel motorcycle. That’s a shame because these look like a ton of fun.

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Tiziano
Tiziano
3 months ago

I rode a KLR for 5 years. It is the most primitive modern bike you can get. It’s a tractor in every sense. It does everything… not well. It is heavy, underpowered and generally rudimental so I can see why it was appealing for the military. However to lower the power further sounds like a recopy for disaster. Let me tell you… it can barely make it in soft sand and jumping would most likely compromise the structure of the bike which is a dual bike, NOT a cross bike. I’d love to own one but this looks like a massive waste of tax payer dollars. All the ones I saw on the internet are on the 500 600 milesage suggesting that they rarely been used even for the 1 tank capacity. 10k is an honest price. Anything north of that it’s non sense.

Mr Sarcastic
Mr Sarcastic
1 year ago

Or Stallone is the Coronel recruits bikers from that Hogs movie to fight along side Ukraine against Ivan Drago motorcycle squadron who has been doing a Ghengis Khan and have two motorcycle gangs square off. Like Rocky meets the Dirty Dozen.

Alistair Gilbert
Alistair Gilbert
1 year ago

check out dieselbike.net UK based also on facebook

Happy Walters
Happy Walters
1 year ago

You forgot the pictures of motorcycles, ships, and helicopters!

https://hips.hearstapps.com/amv-prod-gp.s3.amazonaws.com/gearpatrol/wp-content/uploads/2016/05/Kawasaki-KLR-Gear-Patrol.jpg

A Marine on the flight deck of an amphibious assault ship, with a CH-53E Super Stallion heavy-lift helicopter in the background. This helo is being replaced by the even-huger CH-53K King Stallion.

https://s3files.core77.com/blog/images/2012/09/id_klr_250d8_700.jpg

https://upload.wikimedia.org/wikipedia/commons/thumb/3/34/US_Navy_050418-N-3557N-071_Marines_assigned_to_the_26th_Marine_Expeditionary_Unit_%28MEU%29_transit_the_flight_deck_to_load_KLR_250-D8_Marine_Corps_motorcycles.jpg/1200px-US_Navy_050418-N-3557N-071_Marines_assigned_to_the_26th_Marine_Expeditionary_Unit_%28MEU%29_transit_the_flight_deck_to_load_KLR_250-D8_Marine_Corps_motorcycles.jpg

Two Marines on a flight deck at sea and about to load onto CH-46 Sea Knights. Nicknamed “Phrogs,” the helicopters been replaced by the V-22 Osprey tiltrotor.

The great advantage of diesel engines for the Marine Corps is that it’s a “heavy fuel:” it’s far safer aboard ship. The “naval Services,” which refers to both the U.S. Navy and the U.S. Marine Corps,* try to use JP on every vehicle possible, down to unmanned aircraft systems, in order limit the challenges of transporting, storing, and pumping much more volatile gasoline at sea. Having a common fuel ashore is also a great advantage so that, as you describe, Navy and Marine Corps forces need move and manage a single fuel from ship to shore.

* No, the U.S. Marine Corps is not part of the U.S. Navy. It isn’t. It really isn’t.

Chally_Sheedy
Chally_Sheedy
9 months ago
Reply to  Happy Walters

‘No, the U.S. Marine Corps is not part of the U.S. Navy. It isn’t. It really isn’t.’ – US Navy after Son Thang, or Haditha, or….

Chally_Sheedy
Chally_Sheedy
9 months ago
Reply to  Chally_Sheedy

Yes, I’m aware the US Navy also has its own proud tradition of barely prosecuted war crimes.

But we know they’re not the first to fight.

Chally_Sheedy
Chally_Sheedy
9 months ago
Reply to  Chally_Sheedy

‘No, the U.S. Marine Corps is not part of the U.S. Navy. It isn’t. It really isn’t.’ – USMC after Eddie Gallagh- no nevermind many people are saying he was apparently totally innocent

JDE
JDE
1 year ago

wonder if you could sweet talk a 2009 Track T-800CDI to be imported into the US. It meets all Mercedes Streeter checklists. Diesel Smart 3 cylinder diesel, CVT trans, Off Road styled design. and very weird.

J Edgar
J Edgar
1 year ago

As someone who rides a 2012 KLR and struggles to keep up with his Africa-Twin-riding buddies on pavement, that seemingly small power drop is a deal breaker. These are heavy bikes and already underpowered, so hard pass for me even if I found one on the cheap.

JDE
JDE
1 year ago

I am pretty surprised the MT500 was never made to run on diesel. seems like they were a competitor for the KLR diesel.

Nic Periton
Nic Periton
1 year ago

Have you ever come across the Hero RNT? This thing has been (sort of ) around for ten years, and may or may not exist. At one point it was going to be available as a 2wd hybrid, with an electric motor driving the front wheel. Also it could be used as a generator and the headlamp was to be removable so it could be a work light.

https://autos.maxabout.com/bikes/hero/rnt/rnt-diesel

https://www.eta.co.uk/2014/02/14/the-hero-rnt-a-diesel-motorcycle-thats-a-two-wheeled-pickup/

I would love to know if they are real!

Abraham Smith
Abraham Smith
1 year ago

The fuel situation is not really unusual.
Kerosene and diesel are almost identical (it’s all in the additives and the purity).
Everyone with an old tractor knows that you can get an easier start in cold weather by mixing kerosene.
The DoD has put a lot of effort into making sure fuels are interchangeable

Stones4
Stones4
1 year ago

Having worked around vehicles that used it (and leaked lots of it), JP-8 smells awful, and the smell soaks into your clothes and hair.

MrLM002
MrLM002
1 year ago
Reply to  Stones4

Same with JET-A. In general I like the idea of diesel powered vehicles but gas is so much better to deal with. My favorite pair of gloves just got soaked in diesel yesterday and 1 hand wash with dawn 3 machine washes with Dawn and 1 wash in simple green later and THEY STILL REEK OF DIESEL!

For me the fact that gas evaporates is a bonus, not a downside, but I understand how it’s very different for the Military.

Unclewolverine
Unclewolverine
1 year ago

I had no idea that they made so few of these, no wonder it’s extremely rare to come across one for sale and they command such a high price! It sad that due to current emissions rules there will probably never be a true mass market diesel bike now. I say that Mercedes needs to take a trip to India, stuff every Royal Enfield diesel she can find into a shipping container, and bring them over here for the couple of dozen people who want a diesel bike:)

Gubbin
Gubbin
1 year ago

I always wanted to know where the engine came from. It looks like they converted the KLR engine to diesel by swapping the head and maybe cylinder, and adding an intake and injector. That would be a heck of an engineering project that about 10 people including me would love to know more about.

Doctor Nine
Doctor Nine
1 year ago

I’ve wanted one of these since they came out. Even contacted HDT about acquiring an M2 at one point, and got put on a list. They never contacted me.

Truth is, the physics of diesel motorcycles is inherently compromised, even if this one was serviceable. So, using my sour grapes logic, it’s probably better I never got one.

My next unicorn to imagine, is going to be a PHEV bike. Yeah, that’s the ticket!

Hugh Crawford
Hugh Crawford
1 year ago

“ familiar rabbit hole of diesel-powered motorcycles”
That’s easy for you to say.

So for a geezer that wants a motorcycle that’s as reliable as a hammer and hasn’t ridden in 50 some years ( my last bike was a 57 BSA gold star 500cc hill climb bike , manual spark advance so the kickstart snapped off and I had to push start it ) are the regular KLR650s a good choice?

Mike P
Mike P
1 year ago
Reply to  Hugh Crawford

Yeah, the KLR would be a great bike for you to get back into riding with. It’s been around since the 80’s (with a few very minor revisions along the way) so the used market and aftermarket parts are absolutely huge ans any problems are well documented and pretty easy to fix (Google KLR Doohickey). It’s not really powerful but it’ll get you there – just not really fast. It’s also almost infinitely malleable – take it off-road, commute on it or take it around the world – KLR’s are doing all of those things right now. It’s great for puttering around back roads and exploring all of those mysterious side paths. Here’s a video where one of my favourite YouTube channels try (and fail!) to kill one, in much the same spirit as Top Gear’s Hilux.

https://youtu.be/rvGZS5fqxrk

CSRoad
CSRoad
1 year ago
Reply to  Hugh Crawford

A maybe, it depends on you and your expectations.
It would be on my short list if my riding world changes drastically.

Here’s a recent test:
https://www.youtube.com/watch?v=3CEJmeTrAXM
And an opinion piece:
https://www.youtube.com/watch?v=W7upTCN3M4s

Last Pants
Last Pants
1 year ago
Reply to  Hugh Crawford

The Suzuki Drz 400 is apparently a good option too. Not as heavy or powerful as the klr.

Johnathan Edmund
Johnathan Edmund
1 year ago
Reply to  Hugh Crawford

As the owner of a KLR650, yes. It’s the ultimate “Jack of all trades, master of none” motorcycle. There are better off-road bikes, and faster/more comfortable road bikes, but if you want something you can take down a fire road on Sunday and then commute to work with on Monday, the KLR is an excellent choice.

2007 and older (1st gens) are more off-road focused and a bit lighter. 2008 and up (2nd gens) are more of an ADV style bike and are great all-rounders. 2022+ (3rd gens) are like upgraded 2nd gens with fuel injection.

The biggest perk to the KLR is that they’re CHEAP but reliable. You may end up deciding to swap sprockets or do suspension mods to really hone the bike to your style, but none of that is necessary to enjoy it and even if you did put that money down, it’d still be cheaper than a Tenere 700 or a Africa Twin.

Also, I’ve seen several people in my groups hit over 150,000 miles on their KLRs, so there’s that.

Jason Mason
Jason Mason
1 year ago
Reply to  Hugh Crawford

Honda’s CB500X is an excellent, easy to ride motorcycle that would do well as a re-introduction to riding. Read up on all of the reviews on it. It’s dual purpose (leaning heavily towards the street though) but there are plenty of YT videos of people taking it on adventure riding excursions.

Jason Mason
Jason Mason
1 year ago
Reply to  Jason Mason

P.S. its seat height (32.8″) is lower than the very tall KLR 650 (34.3″), making it easier to flat foot if your inseam isn’t cooperative. However, for 2023 there is a KLR 650S, a lowered version of the standard KLR with a seat height of 32.1″.

Fawgcutter
Fawgcutter
1 year ago
Reply to  Hugh Crawford

Back in 2000, rental agencies in Alaska offered them to tourists as an inexpensive off-road capable bike. I rode my bike to Coldfoot and encountered a pair of them at the rest stop there. If the rental agencies considered them reliable for dirt roads then, I think the newer version would be more reliable now. (The tourists were amazed seeing me there on a Harley, though.)

Hondaimpbmw 12
Hondaimpbmw 12
1 year ago
Reply to  Hugh Crawford

A little tall if you are inseam challenged like me. Not exactly at 255lb KTM, but can be made to ride smoothly and reliable as a stone axe. The bugs have been worked out. Fast enough. My B-I-L has a set of 18” wheels w/ street tires for commuting to work, then throws on the 21 ‘& 17” wheels w/ dirt rubber for fire trails. Not a fan of single track.

CSRoad
CSRoad
1 year ago

Well Kawasaki would probably make them if there was enough verifiable demand and they could get it by Euro5 emissions somehow. The KLR650 is still current and Kawasaki makes Diesel engines of all sizes, they could do it if it was worthwhile for them. I just can’t see it happening at this point in the game.

The KLR650 is a great bike though, just feed it 87 octane 10% corn squeezing gas and it is happy. (-:

Gene1969
Gene1969
1 year ago

These were always an intriguing motorcycle for me. I wanted one like I wanted a BMW. I never got either but thank you for the memory.

Justin Short
Justin Short
1 year ago

Nice, but, omg what a nightmare for parts !I

Mr Sarcastic
Mr Sarcastic
1 year ago

This is an amazing build accomplishment. But i have to ask exactly what are they of use for? They arent armed, and if the rider does get into a fight that bike just sits there waiting to be fired upon. The numbers dont say fast acting seal or ranger team. Great mpg but unfortunately their refuelers cant keep up and they have to return. Most military bikes were for behind the front lines delivery of new orders.

Doctor Nine
Doctor Nine
1 year ago
Reply to  Mr Sarcastic

They were intended for reconnaisance and long distance unaccompanied patrols in the desert. The range was the reason for the design, not the speed.

Mr Sarcastic
Mr Sarcastic
1 year ago
Reply to  Doctor Nine

Thanks wasnt in the original column. I love learning things. WWII more my area.

Jason Vorhan
Jason Vorhan
1 year ago
Reply to  Mr Sarcastic

Dude, have you never even seen the documentary MegaForce?

Stephen Bierce
Stephen Bierce
1 year ago
Reply to  Jason Vorhan

Let’s get low-budget Hollywood to remake that, _The Wild Geese_, _Nam’s Angels_ and _Born To Ride_ all at once! A mercenary company recruits some convicted biker anti-hero gangbangers out of the for-profit prisons for the carnage in Ukraine’s occupied zones. The bikes and bikers are dropped behind the Russian front lines and let run amok dynamiting railway bridges and calling in airstrikes on Russian soldier camps.

Mr Sarcastic
Mr Sarcastic
1 year ago
Reply to  Jason Vorhan

No

Frankencamry
Frankencamry
1 year ago

Any of these vehicles with small enough production numbers that they’re unlikely to have “1 per military museum or well heeled collector” availability after service is going to be priced above its functional value, unfortunately.

Still, fun to hunt.

BlueCruiser
BlueCruiser
1 year ago

The reason I don’t want land is because of government auctions. The reason I want land is because of government auctions

Crank Shaft
Crank Shaft
1 year ago

Alas, I feel your late to the party pain, but feel compelled to mention that you can probably find enough other vehicle distractions to allay you agony. 🙂

Toecutter
Toecutter
1 year ago

The speedometer seems to indicate a 7,500 rpm redline for this engine. If true, that is an extremely high cutoff rpm for a diesel. Perhaps the tachometer was intended for the gasoline variant of the bike?

Drew
Drew
1 year ago
Reply to  Toecutter

I suspect that the tach is just the same as the gas one, as you guessed. The limited info I can find about this one indicates maximum power at 3300 rpm, while the gas bike rates max power at 6500.

Rust Buckets
Rust Buckets
1 year ago
Reply to  Toecutter

I read that it made 30hp, out of a 600cc indirect injection naturally aspirated diesel. That’s a whole lot. It must rev fairly high for a diesel to be doing that.

Acid Tonic
Acid Tonic
1 year ago
Reply to  Rust Buckets

Agreed my 1.9 turbo diesel had 90hp stock from 3 times this engine size with direct injection and boost.

Also diesel engines can rev the fuel supports it. Most engines have such heavy duty parts the redline is from mass not flame speed.

My 1.5 na diesel would scream with a 6500rpm fuel cut. With enough timing advance and a lean enough mixture to burn before the next cycle diesel can male power up to around 7000rpm.

My 1.9 with an HY35 doesnt even make more than 2psi boost until it spools at 4200rpm and pulls hard to 6600. It would rev faster but you know my pistons are twice the mass of a chevy 350 piston and makes them look like little kid toys if side by side.

Bigger diesels dont rev beyond 3000rpm, some redline at 2000rpm. Big ones in ships redline at 800rpm. Its mass not the fuel.

Peter Andruskiewicz
Peter Andruskiewicz
1 year ago
Reply to  Acid Tonic

Eh, its a bit of both mass and fuel, or more precisely the fuel injection method. The average flame rate for pre-mixed gasoline is dependent on the in-cylinder turbulence, which is fairly directly linked to the engine speed. Therefore, gasoline and air’s combustion rate is a pretty constant number of crank angle degrees (NOT time) for a given intake pressure, assuming there isn’t a lot of EGR and that the air-fuel ratio is near stoichiometric (which it almost always is, except for knock mitigation & exhaust valve/turbine thermal protection at high loads under boost).

In comparison, a diesel engine’s burn rate at high loads is strongly dependent on the fuel injection rate, subsequent mixing with air and vaporization, most of which are more dependent on absolute time instead of engine speed. First, a not-so-quick primer on the diesel combustion process. First, fuel is injected at very high pressures typically about 10-20 degrees before the piston reaches TDC, lasting for for 20-40 crank degrees depending on the engine. Once the fuel is injected, it has to break apart into lots of little droplets with lots of surface area for their volume, vaporize, find oxygen, and then combust. This is pretty easy for the first 20% of fuel injected – it’s getting sprayed into a chamber full of high pressure & temperature air that’s relatively stagnant, so the tip of the spray breaks apart easily, vaporizes due to the compressed gas temperature, and finds plenty of willing O2 dance partners. All this takes a bit of time, but then the first 20% or so goes up in a big spike (typically called the pre-mixed ignition spike). The next 60% of fuel is injected into the already burned column left by the first 20% which is already moving away from the injector which hurts droplet break-up, is hotter which helps vaporization, but is oxygen deficient, so even though this spray is vaporized it still needs to travel outside the burned jet left behind by the first 20% before it can find the O2 needed for combustion. When it finds the O2, it already has an ignition source from the first 20%’s pre-mixed ignition, so the combustion rate is dictated by the time it takes for break-up, vaporization, and mixing with O2 to take place. All of this takes more time (relatively anyway) than for the first 20% of the diesel jet, or for easily vaporized gasoline that was injected much earlier in the cycle in the SI engine. The last 20% is the hardest, especially if you’re at the typical high-load AFRs of about 18:20 parts air/part fuel that diesel engines run at to avoid too much soot. This last bit of fuel needs to work even harder for the droplets to break apart and find O2. By this point almost all of the O2 in the piston bowl has been consumed, so some portion of this last 20% of the fuel needs to make its way out of the bowl and up into the squish-region where conditions are cooler (very little direct combustion and higher heat losses due to the close proximity being “squished” between head and piston near TDC). Fortunately, as the piston moves downward, the gaseous contents of the piston bowl are drawn up into the volume above the piston where the remaining O2 is, but at this point its already getting towards the middle of the expansion stroke where any additional pressure that’s gained by combustion can do less work mechanically. Also, the temperature is getting colder due to the expansion, which starts to slow down any remaining reactions that are underway, freezing the intermittent species formed during combustion as CO, smaller unburned HCs, and carbon particles. As this continues, the unburned HCs preferentially agglomerate on the carbon particles, making soot that is even less likely to burn even if it finds O2 due to the colder in-cylinder temperatures. Even if it does burn, the piston’s continuing to approach BDC, so the amount of useful work it can do on the piston is minimal, so any energy just goes into heat that’ll be wasted out the exhaust (the turbine can harvest a little bit of this, but only if the wastegate is closed and the engine can handle more boost).

A modern turbocharged common-rail diesel (which this bike is not!) will increase the fuel pressure at high loads more quickly than a mechanical injection pump to allow the time that the injector is open and spraying fuel to be a bit shorter, but there are diminishing returns here since you still need to deal with fluid momentum, compressibility, losses through very tiny injection holes etc. In the end, the injection event is still typically about as long gasoline combustion event (at least with OEM rail pressures and injector sizes designed to run well over the entire engine speed range). Higher injection pressures also help the droplet break-up & vaporization, and increases the spray penetration, but this only helps as long as you aren’t already spraying the bowl walls. Multiple short sprays can also help by essentially pre-mixing the first 20% of fuel, allowing a gap for fresh air to fill in, then spraying into that gap. Modern injection hardware can do this, but it’s very tricky to get right, costly, and requires a LOT of calibration once you get into 3, 4, or 5 pulses. This also elongates the entire injection event, so there are diminishing returns since the tail of combustion gets later in the cycle when the piston can do less work. High levels of air swirl (bulk rotation of the air about the cylinder’s axis, induced by the intake port design) also helps by spinning fresh air into the sides of the “flower” of injection jet plumes to increase mixing with O2; this option does introduce better high-engine-speed performance since the amount of swirl is dependent on engine speed, but often the effect is limited because generating that swirl hurts the volumetric efficiency – you need more boost to push the same amount of air in, and you need to pay for that boost with back-pressure, which increases the pumping losses, internal level of exhaust gas residual, and heat losses to the cylinder walls due to the swirl.

So, in short (or TL-DR; I won’t blame you, this turned into a bit of a novel), while you can make a diesel engine make power at high speeds, you’ll be doing so very inefficiently, with lots of soot, because many of the processes that need to occur in a CI diesel engine are more time-dependent than engine-speed-dependent, while the main driver for combustion speed in SI gas engines is engine-speed-dependent.

Stones4
Stones4
1 year ago

Having flashbacks to calibrating a diesel with 3-4 pulse injection. What a nightmare that was

Mike P
Mike P
1 year ago
Reply to  Toecutter

That’s definitely just the stock instrument cluster from the commercial KLR. In fact, aside from the engine and the aftermarket fuel tank and ‘fairing’ the bike looks almost bone stock.

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