Some thoughts on adding fuel, boost and propane to a diesel



Oh gosh, where to start...? ;)

I love this (general) topic, so all my friends out there please forgive me if I run a little long here, eh? :roll: Also, I am suffering from jet-lag. I just got home from a couple of weeks of shuttling Grunts and their stuff from Europe to the Sand Box, and am now home on the West Coast wondering why the sun went down when it should have been coming up, etc. So please forgive me if I come across as even more goofy than normal. 8)

Except for some early diesels that ran on solid hydrocarbons (coal dust, sawdust, peanut shells, etc), no diesels since have been designed to run by detonation. They all run using the same controlled flame front idea as spark ignition engines, it's just that their method of igniting the fuel differs. And in fact, CI engines (technically these are compression ignition engines...the fuel is called "diesel") can be configured to run on any liquid hydrocarbon from alky to axle grease. That includes gasoline, kerosene, methanol, jet fuel, used McDonald's frying oil, diesel, etc. Diesel is the fuel of choice for a variety of reasons, the most important probably being that diesel acts as a lubricant for the mechanical high pressure injector pump typically used in CI engines. Diesel is also very stable, doesn't evaporate readily, and is quite safe to store and handle compared to gasoline. In prior decades, diesel was largely a by-product of the distillation process used to make gasoline, which gave the refineries another product to sell (rather than simply burning it off). Today's refining technologies aren't nearly so dependent on distillation, though, so this is no longer much of a factor. In fact, there is little to choose from between gasoline and diesel for a modern catalytic-based refinery, so they just refine to their immediate market.

As 84_250 explains above, CI engines inject their fuel into air that is already hot enough to ignite the fuel, so combustion starts more or less immediately. Picture it like this. Air enters the cylinder during the intake stroke and the intake valve then closes. The piston now rams upward in a few ten-thousands of a second, compressing the air and heating it at the same time (the very act of compressing air heats it) to more than 900 degrees F. A few degrees before TDC, the injector squirts fuel into this very hot air and the fuel ignites, reaching maximum pressure (this is the point of max torque for the engine) at roughly 15 degrees after TDC.

So far, we are just considering a normally aspirated diesel engine, so let's add more fuel, a turbo, nitrous, and propane in sequence. Diesels make more power to a point of "heavy smoke" as one injects more fuel for the available air. That's why you can melt the piston of a NA diesel by cranking up the fuel delivery. The added heat and pressure inside the combustion chamber can literally melt an aluminum piston, which is why many "old timey" diesels used cast iron pistons -- they could crank up the fuel delivery without destroying the piston. Modern diesels have aluminum pistons and use a turbo to keep temperatures in check. They make more power by adding air and fuel for clean, low temperature combustion without nuking the piston.

Adding air from a turbo or nitrogen and oxygen by injecting nitrous oxide accomplishes the same thing. One is leaning the mixture in a fuel/air ratio sense. Leaning the mixture lowers the temperature of the combustion process in a CI engine. Of course, a turbo adds compressed air, which leads to a temperature rise in and of itself, but not nearly so high as adding dangerously large amounts of fuel without more air. So I am not surprised that the added nitrous made little additional power in this test -- you can accomplish the same thing by simply adding a little turbo boost. The bottom line is that adding more air (by more boost or nitrous or both) allows one to safely add fuel to make more power without melting critical engine parts. Naturally, as one makes more power, additional cooling capacity has to be added as well or all this good work will come undone. We haven't turned physics on its head here after all... ;)

So how does adding propane to the mix help us make more power? Easy, adding a little propane allows us to burn the injected diesel more efficently. This results in more power in and of itself, although there is a little additional power from the propane (but not much if done correctly).

Here's how it works, by fumigating (adding as a gas) propane upstream of the turbo, we assure the propane is evenly distributed to each cylinder by the mixing action of the turbo blades. Propane has an ignition temperature of about 1000 degrees, so won't self-ignite as the piston sweeps up on the compression stroke. However, as soon as the diesel is injected, it ignites, which ignites the propane in turn. The flame front from the propane helps boil off and ignite the tiny diesel proplets being injected into the cylinder, producing more complete combustion and more power per unit of diesel injected.

There you have it, a quick and dirty on some of my favorite engine subjects... :D

Some additional thoughts, though. Simply making more power won't usually negatively affect an engine, so long as cooling and lubricating requirements are adequately looked after, and RPMs are kept in the safe zone. I imagine that the Izuzu can make plenty more grunt without puking its guts out. More boost and fuel, with appropriate chip programming to account for it, and one can have all the reliable power one wishes for. I'd skip the nitrous, though. It's not "dangerous". It just isn't needed. Simply add more boost and fuel! Why add something that needs frquent replenishing when what you already have on board will work just as well, and doesn't require a run to the speed shop every few weeks?
Umm, like I said...a little more goofy than normal. :roll:

This should have been a reply to SR's Do you mash the throttle or just pull the pin? topic. I guess I hit the New Post instead of Post Reply button. Oh well, maybe someone can make it right... :LOL:
Not yet, but I recently bought a Turbo Cat 3208 to replace the NA engine I had, and propane is definitely on the agenda. :D
This is plenty good enough to stand alone as a topic, Stan - why worry about continuity? In fact, it has cleared the way somewhat. :D
Yeah, bouncing back and forth can mess up your bio clock, especially if direction changes occur several times quickly.
Landing before you take off, two birthdays in the same 24 hours, once had a week with three Mondays or so it seemed.
Just keep the number of your take offs and landings the same, OK.
Thanks for clearing that up, Stan :)

So how come there are such widely varying Compression Ratios for Diesels? Thats another concept that has escaped me, I've always thought that it needed a high CR in order to increase the charge temperature (P*V = n*R*T) to exceed the fuel's flash point..
Hey Disco... :D

Commercially available diesels do come in a bewildering array of compression ratios, I'll grant, but there is a rational explanation for this.

Older diesels, like my 1980 1.6L VW Diesel Rabbit (Golf), had engines which used indirect injection and glow plugs. The fuel was injected into a small cavity in the head above the piston instead of directly into the cylinder. Because this cavity was somewhat isolated from the main combustion chamber, the engine required a higher compression ratio to raise the temperature inside the cavity high enough to assure reliable ignition. And in fact, the engine couldn't do it when starting cold, in spite of its 22:1 static compression ratio, and hence had to use a glow plug to heat the cavity for about 15 seconds before the engine would start in cool weather (below about freezing). For this reason, indirect injection diesel engines typically have CRs ranging from 20:1 up to about 25:1.

Direct injection engines, OTOH, much more efficiently heat the air charge on the compression stroke, and will readily start in cool weather with no glow plugs and CRs as low as 14:1. My new (2002) Golf TDi is a good example of this. A few weeks ago my wife and I hopped into her Golf (with her in the pilot's seat) with the temp just a few degrees above freezing. As she always does, she simply turned the key to the start position (Golfs do have glow plugs for use in extreme climates) and the engine fired right off without any hesitation. I cringed inside at this abuse (our old Rabbit would have choked at this treatment), but technology does march on... :roll:

Direct injection diesels range from 14:1 up to about 20:1, with turbos predominating at the lower end, and NA engines at the upper. This is due to the long proved fact that thermal efficiency of any piston engine rises with CR until pumping losses and other friction sources overtake it. A good example is the venerable Cat 3208 engine. In its NA form, this 10 liter direct injection industrial diesel comes rated at 125, 175, and 210 hp, with the CR rising from (IIRC) 14:1 to 16:1 and to 18:1 static. It's also available in turbo form with 225, 275 and 350 hp available, at roughly the same CRs.

Since a properly tuned diesel can't detonate (even after adding nitrous), it's simply a matter of more compression, more boost, more fuel, and better parts to make more hoss-pressure. Don't ya just wish gas engines were that straight forward? :roll: ;)

OTOH, I have seen lab engines (made by mech eng studs) that would start and run reliably at 12:1. They weren't very powerful, but they certainly worked.

Another thing to keep in mind is the cetane of the diesel fuel. For those who aren't familiar with cetane, it is almost the opposite of octane for gasoline. Octane is a measure of gasoline's resistance to auto-ignition (detonation), the higher the better. Cetane is a measure of diesel's willingness to ignite (auto or not), also the higher the better. Diesel with a high cetane number, say 50 or higher, is much easier to ignite and is thus better suited for lower CR diesel engines. Higher CR diesel engines can tolerate lower cetane fuel, all other things being equal.

Sorry about the long answer to a short question, but I hope I covered what you were asking.
I had been running my Trooper diesel on 40 cetane (standard#2) diesel fuel all summer, then got fuel up in Maine, which was 50 cetane (diesel #1). That thing came alive, but got horrendous mileage. I think it dropped to somewhere around 20mpg. But, it started easier, and didn't make as much black smoke. One thing to keep in mind, though, is if you are going to tune a diesel, and turn it up a bit, is to retard the timing of the injection pump. I know this doesn't make much sense, compared to a gasjob, but to prevent a burnt piston, and get the full benefits of your mods, retard the timing.

At the risk of boring you guys to death, I want to expand on some thoughts about pumping and friction losses, compression ratios and the turbocharging of diesel engines.

First, since diesels don't have a throttle choking off the intake air stream, they breath with almost no intake restriction. To picture how important this is, compare trying to breath while jogging through a straw versus with your mouth wide open.

Second, a primary cause of friction inside engines is the friction of the rings against the cylinder walls as the pistons go about their up and down business. The relationship was laid out long ago by Stribeck as [(V*vpr)/Wpr]*n, where

V = oil viscosity
vpr = ring velocity
Wpr = ring load (tension)
n = a constant to compensate for oil viscosity

For those who are math challenged, what this really means is that for a given ring package and oil viscosity, the friction rises exponentially with engine piston velocity (RPM). And this is an area where diesels really shine, since their RPMs are kept relatively low compared to similar displacement gasoline engines.

Recall that the thermodynamic efficiency of an engine increases with compression ratio? That's a good thing of course, but there is a downside. Higher compression ratios introduce additional stresses on engine parts, make starting the engine difficult, and raise NOx emissions, among other negatives. So, engine manufacturers are tempted to lower compression ratios in a search for an ideal compromise. And that's where a turbo saves the day.

A turbo allows for a lower static compression ratio, yet allows the engine to make power upon demand -- the best of all worlds. Let's look at an example. A direct injection diesel with, say, a 20:1 CR is easy to start, has low stresses, and lower emissions compared to the same engine with a 25:1 CR. Yet when more power is needed than provided by the engine running unboosted, the driver has simply to step on the pedal to add boost and make the required power. The engine in my Golf is a good illustration of this. Running along at 55 MPH on the flat, the engine is running unboosted and putting out about 12 hp. Yet if I need to pass someone, the turbo screws down to give me an almost instant 18 PSI boost, raising the CR from 19.51:1 to an amazing 42:1 effective compression ratio, and raising the hp to 90. No wonder it can deliver 50 MPG at 75 MPH!

Now, while these comments have been aimed at diesel engines in particular, there is little here that doesn't apply to gasoline engines. With all of this (and more) in mind, I hope you now understand why I am progressing as I am with my Mustang project. I acquired a late model, low compression 250 engine, and am turbocharging it to for both economy and power at low RPMs, which brings me to my last point. Does anyone out there have a 2.23 rear end? I read recently that these were available on the base 170 and 200 models in early Mustangs, and I would like to get one for my car.

Thanks! :D

You are facing two different issues here. First, the cetane rating of diesel is independent of whether is is winter or summer formulation. Cetane is purely a measure of the fuel's willingness to ignite (to keep things simple). During the winter, many States require fuel distributors to dilute diesel fuel with kerosene (and call it diesel #1) to make it flow better. Unfortunately, kerosene has less energy content than pure diesel, so your mileage falls off during the winter.

Try buying "premium diesel". Premium diesel fuel must meet federal standards for several important criteria, including cetane and energy content, and should recover your fuel mileage.
Cetane is directly linked with the energy content of a fuel- you and I are arguing the same side. K1 has a very high cetane rating, which means it ignites much more easily, but does not contain the same energy content as a lower cetane diesel fuel.

Equating cetane number and energy content of diesel fuel is a common misconception. Here are a couple of useful definitions snagged off the web.

Cetane number is a measure of the ignition quality of diesel fuel. The higher the cetane number the easier the fuel ignites when injected into an engine. Cetane number is determined by an engine test using two reference fuel blends of known cetane numbers. The reference fuels are prepared by blending normal cetane (n-hexadecane), having a value of 100, with heptamethyl nonane, having a value of 15.

The energy content of diesel fuel is its heat of combustion, the heat released when a known quantity of fuel is burned under specific conditions. In the U.S., the heating value is usually expressed as British thermal units (Btu) per gallon at 60°F. Btu per gallon is directly proportional to density of the fuel. Density is the weight of a unit volume of material at a specific temperature. The unit used to report density in the U.S. is API Gravity. API numbers typically range from 10 to 70. As the numbers increase the density and the Btu of the fuel decrease.

Exxon maintains a good, non-technical diesel fuel FAQ at ... ls_FAQ.asp
You are boring us to death on this diesel/distilate thread.
Many thanks for the information but shouldnt it end here.
backlash":34ofogbt said:
You are boring us to death on this diesel/distilate thread.
Many thanks for the information but shouldnt it end here.

Guilty as charged... :roll:

OTOH, you were warned... :nod:

54Ford":34ofogbt said: all my friends out there please forgive me if I run a little long here, eh?
But I bet he does, because it does not matter how uninteresting this is, it is still fascinating.
I'd be the first in line if someone would offer a nice turbodiesel truck package that didn't run into the 40K range. I really love hitting europe, as they have some of the coolest TDs around- in all types of platforms.

I had a TD Land Cruiser down in Peru, and it was awesome- despite buring near-candle oil for fuel. Torque is good!I really hope TDs make a big push here- with modern DI electronics available, they might meet emissions standards of the future.

DI should be able to make super-lean gasoline engines, too- but the patent holders are squabbling over who gets the right to sell to everyone, and who was first. OMC bought FICHT engine technology from germany, which is a DI system that injects fuel right at the piston. At low speeds, the spark plug (which is in the air/fuel spray pattern) fires the mixture as it goes by, resulting in a flame front hitting the piston just before TDC. I tested these engines for the military, and they were 34% more efficient in our application. The only real problem was the expense of the injector unit- about 1/4 of what the whole powerhead cost.

Oooops- I guess I'm boring you too. Nice thread, Stan. I guess you Galaxy guys have lots to talk about when ya fly for 12 hours, eh? ;)
I should point out that Noel is Australian.

It has long been a national pastime to stir Americans. The results are often funnier when compounded by misunderstanding... "That's so offensive, saying we're like septic tanks." :roll:

I might post the duck shooting joke on FSP sometime...

Regards, Adam.
Not to worry Addo and Noel. I may be a boring guy, but at least I'm not a humor-free zone... ;) :roll:

Hey Jeff, 14.4 hours from Spain to the Left Coast the other day...unrefueled. :shock:

Of course, had I followed the family tradition and joined the Navy, it would have taken me 10 weeks, refueling every night. :eek: