Whay is 14.7:1 air fuel not important in a Diesel?

[Agent WD-40]

Ok, so I know diesels don't have any butterfly plate to control air intake, or even a throttle body. I assume they just suck in as much air on each stroke as they can. Does this mean that they will fill the cylinders will different volumes of air at different RPM's, much like a gas engine will suck in different amounts of air at WOT?

If the engine controls the amount of fuel injected instead of the amount of air allowed to enter the engine, I assume the air fuel ratio will not be 14.7:1. Anybody know what the range is? So to change the amount of power you get out of the engine you squirt whatever amount of fuel you want into the cylinder and it will combust it and get the power out of the fuel. If you squirt little fuel you will get little power, more fuel more power.

First, just so I'm savvy, let’s review 14.7:1. This is the stoichiometric value of air and fuel. Having this ratio will yield full combustion (in theory) and thus maximum power. Changing that ratio will yield less than optimum power. Is this all correct?

Would there be an upper limit on how much fuel you can squirt into the cylinder to yield max power? Is their an idea number for diesel like 14.7:1 for gas?

Thanks for the help.

[Sanosuke]

Read this page to learn how diesel engines work:

http://auto.howstuffworks.com/diesel.htm

Enjoy reading the article.

Sanosuke!
Return to Japan - Spring 2004 (Bonus: Singapore)

[Agent WD-40]

Thanks for the link. That was the first place I looked, but it didn't go into A/F ratios and such.

[Sanosuke]

Originally Posted by Agent WD-40

Thanks for the link. That was the first place I looked, but it didn't go into A/F ratios and such.

Air-Fuel ratios are not used in Diesel engines -- the air is what ignites the fuel mixture.

Quoting the page as follows (Source: http://auto.howstuffworks.com/diesel1.htm)

"Gasoline engines generally use either carburetion, in which the air and fuel is mixed long before the air enters the cylinder, or port fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder. "

"Note that the diesel engine has no spark plug, that it intakes air and compresses it, and that it then injects the fuel directly into the combustion chamber (direct injection). It is the heat of the compressed air that lights the fuel in a diesel engine. "

There you have it, the air compression is what detonates the fuel -- not the spark of the fuel like it is in gasoline engines.

Sanosuke!
Return to Japan - Spring 2004 (Bonus: Singapore)

[Agent WD-40]

Air and Fuel are still in the combustion chamber. There will be a particualr volume of air and a particular volume of fuel in the combustion chamber. I a gas engine you want to keep it close to 14.7:1, in a Diesel it doesn't seem to need to stay at that ratio. Perhaps that don't worry about the volume of air to fuel in the combustion chamber with Diesel. The air and fule still need to mix in a Diesel, or there would be no combustion. I know they have creative ways of doing this.

I would imgaine that there would be a point where you would inject too much fuel directly into the combustion chamber and begin to loose power, much as a gasoline engine runs rich. Perhaps I'm failing to understand how fuel to air ratios on a gas engine work, thus my reasoning is off. But I still think what I'm saying makes sense. I'm failing to understand something somewhere. Someone is going to need to point out where my reasoning went awry so I can get the whole train of thought back on track.

[Wstar425]

I can't answer your question about air/diesel fuel mixture directly, if at all, but will attempt to share some intel on the subject. By the way, I've been driving semis for 27 years and have never heard anyone be concerned about this question, or figure; though I am a driver and not a mechanic. Not exactly sure where you are going with it. The 14.7 figure sounds about like a compression ratio for a diesel to me. The "How Stuff Works" site gives a basic overview of how a diesel engine functions but doesn't really get into many specifics.

Most diesel engines are turbocharged. This functions something like a supercharger, but less efficient, less responsive, but also less parasitic on the power. The turbo operates off of a fan spun by the exhaust gas. The exhaust gas does not return into the engine (untill 10/02 EGR engines anyway)
http://www.cat.com/products/shared/t...cert/index.htm
but is kept seperate. So, the faster the rpm, the more air is forced into the cylinder. There is somewhat of a lag time on the turbo operating, more fuel, faster rpm; faster rpm more air from the turbo: more air from the turbo, more fuel: more fuel, faster rpm. and so on........again, I can't give you an ideal air/fuel mixture number, if one even exists. Please remember that I am NOT a diesel mechanic. If there is one on here, please feel free to correct me as needed. (maybe all of it!

Diesel engines are direct injected, by at least two different methods. Some engines (Cummins, maybe not all) have the injectors in the heads with a fuel line through the head sort of past each injector. The injector is opened with linkage from the valve train. There is constant pressure in the fuel line and when the injector "opens" fuel is sprayed into the cylinder. The pressure in the line (rail pressure) can be increased by turning up the pump. Certain limits exist where you would need to change the turbo and some other parts if you went crazy with turning up the rail pressure. Most of the newer, electronic controlled engines have the capability to make some horsepower changes through the engine computer and a laptop with the correct software. Detroit Diesel was sort of the pioneer in this technology.

Other diesel engines (Caterpiller, IH, John Deere, and others) have the injectors in the side of the head and have a fuel line to each individual cylinder. This fuel pump is much more complicated and precise than the previously mentioned style. The pump itself determines exactly when to send fuel through each line and how much. You will notice if you look at this type of engine that the cylinders closest to the pump have fuel lines that sort of go all over the place instead of running in a straight line. This is so that each line is exactly the same length. All lines are the same length as the one going to the farthest cylinder from the pump. This way the same amount and pressure of fuel is sent to each cylinder.

As the rpms increase the turbo spins faster forcing more air in the cylinder. At the same time more fuel would be injected into the cylinder to increase the rpm even further. I don't have any idea if this ratio is the same, at say idle and full throttle. I guess my thought would be that it would be a lot larger percentage of fuel at full throttle than at idle, but don't know for sure since you are also jamming a lot more air into the cylinder with the turbo.
(No brainer?)

You might have noticed some trucks belching out huge amounts of black smoke, especially starting out or pulling a big hill. That is mostly unburned fuel,
either due to the pump turned up and providing more fuel than can burn in the power stroke, possibly bad injector(s) spraying a poor pattern, blown turbo, or any other number of things not right. The idea is that you get a little more fuel into the cylinder than you burn, that way you know you have as much as you possibly can burn. I don't know if you can be "too rich" and actually lose power or not. I guess you could, taken to an extreme it probably wouldn't run at all. I do know that usually when they are blowing black smoke they are running very good, or terrible; at least till they blow up. With the price of fuel these days, and states like California that have opacity regulations, you don't see the trucks having much visible exhaust anymore. Can't afford to have that much fun any longer if you want to survive. California used to give you one freebee, but the next fine was in the thousands of dollars so you didn't want to do that real regular like.

I know I didn't answer your main question but hope this helps somehow/what. Again, feel free to correct my misguided thinking as needed. I'm jsut a sipmle trcuk drvier; and sometimes that is debatable.

Happy Hauling,

Doug

[KLS]

There are two main internal combustion engine cycles...
Otto cycle and Diesel cycle.

Nicholas Otto, 1831-1891, invented the spark combustion four stroke cycle engine. This is the basis for our gasoline engines. Fuel and air mix outside the combustion chamber, are drawn into the combustion chamber, ignited at the correct time by a spark plug, and the burning and expanding gas pushes the piston down for a power stroke.

Rudolph Diesel, 1858-1913, invented the compression ignition engine. Air is compressed inside the cylinder until it is hot enough to cause the fuel to burn. Fuel is sprayed (injected) into the cylinder at the correct time and for the correct duration to give the desired power output. The burning and expanding gas pushes the piston down for a power stroke.

Nope, I don't know why air-fuel ratio isn't considered on a diesel. The only time it's a problem is if the engine is overloaded or there's an injector or turbocharger failure, and black smoke is a result.


Ken

[makana81]

First thing to understand is that a Diesel engine will have it's max air input no matter what the fuel input is. Also, Diesel engines are not limited indirectly like gas engines, which use a butterfly valve which limits air intake, which limits fuel, which limits speed.

The engines speed in a diesel is limited only by the amount of fuel injected into the engine cylinders. Therefore, the engine will have enough oxygen to burn, and the engine will attempt to accelerate to meet the new fuel injection rate. As a note, an unloaded diesel engine can accelerate at a rate of more than 2000 rps. This is why diesel engines require a speed limiter (aka fuel governor) to control the amount of fuel being injected into the engine.

Finally, the air/fuel requirements for gas engines are 17.1-15.1 for economy; 14.7 for all-around; 14-12.1 for power. Diesel engines have practically no basic guideline to follow like gas engines do with the air/fuel mixture; more gas, more go---more compression, more gas, more go, more efficient.

Hope this clears a little more of your questions.

[lelandstanford]

I am kind of confused on this issue of how much fuel is injected into the combustion chamber.

If air/fuel ratio of a gasoline is too lean, pistons may get damaged from high temperature.

So, besides the issue of emission, why can't the cooling system remove the heat?

Since a diesel engine almost always get full dose of air in every intake cycle regardless of RPM. Why won't the pistons get damaged if idling (little fuel injected).

[Sanosuke]

This url should explain why diesel engines don't get messed up at idle:

http://www.sh015a7585.pwp.blueyonder.co.uk/diesel.htm

The pertaining quote comes from the url above for those who don't bother to read the whole thing:

Apart from injecting fuel according to the operator's demand for torque, the diesel fuel system must perform a secondary function, that of governing the engine. The pumping inefficiencies in a petrol engine increase with speed, imposing an upper limit on the latter and stabilizing idling. If the idle speed drops the pumping torque reduces, causing the engine to speed up again, and vice versa. Because this effect is not nearly so pronounced in the diesel, it has a natural tendancy to stall at low speeds and, if no load is applied when fuel is injected, to accelerate up to speeds which can destroy the engine. Thus, instruments known as governors are attached to the fuel control device in pumps or injectors, in order to achieve a stable idle speed by adjustment of the fuelling level, and to cut off fuelling over the maximum safe speed. Traditionally, governors consisted of mechanisms using fly-weights, levers and springs or pneumatic or hydraulic valves. However, the low cost and enormous flexibility of digital electronics combined with increasingly demanding emissions limits, fuel economy and drivability targets, mean that electronic control is now predominating.

Hope this explains how the diesel engines don't self-destruct at idle.

Sanosuke!
Return to Japan - Spring 2004 (Bonus: Singapore)

[KLS]

Leland,
Gasoline engines usually don't have cooled pistons. The heat of combustion on the piston crown is passed through to the bottom side and into the air in the crankcase, and also through the piston rings into the cylinder wall and thence to the coolant. Gasoline engines running lean at idle won't overheat...gasoline engines running lean at high power levels will soon melt the aluminum piston crowns. Most (all?) diesels have cast iron pistons, and bigger ones have replaceable forged steel piston crowns bolted on.

Many diesel engines have cooled pistons, either an oil spray on the bottoms, or oil circulating in cooling passages in the piston, or in a few cases, cooling water circulating in cooling passages in the piston. On a diesel, as the load increases, the air-fuel ratio decreases. The beginning of injection is later at light load and earlier at heavy load. The ending of injection is much later at heavy load.

Makana,
A turbocharged diesel engine will get more air as the engine's load increases and the hotter exhaust gases drive the turbocharger faster. More load, even at the same rpm, and more air is pushed into the intake. This is really noticable on a diesel generator where the speed is kept constant, and when more load is added the turbo can be heard to speed up. The turbo boost gauge shows this, even on my turbochaged Volvo running on cruise control...same rpm, come to a slight hill, and the boost increases.


Ken

[Wstar425]

Sanosuke,

An excellant article and web site, I couldn't have said it better myself; and didn't! Will that 108,000 hp Sulzer Diesel fit in my Tundra if I make the picture real small? At 100 rpm, that's awesome, wonder how much torque that baby has?

Happy Hauling,

Doug

[Sanosuke]

I did some calculating:

The Formula for calculating full load torque at 100rpm is:

HP * 5252 / R = Torque (lb-ft)

108000*5252/100 = 5,672,160 lb-ft of torque at 100 rpm!

Now thats serious work there.

Sanosuke!
Return to Japan - Spring 2004 (Bonus: Singapore)

[KLS]

All those large diesel engines are run on the test bed before they get installed in the ship. They're tested at 100% for a certain period, then at 110% of rated load for another period of time. The full output is developed into a dynamometer...a device that absorbs the full output, measure the torque, and from that plus the rpm the horsepower is calculated. Needless to say, cooling the dyno is a major consideration...the dyno is hydraulic (not friction) with a way to measure the torque level.

Think of the sound of a 100+,000 hp diesel engine inside a building--four turbochargers screaming, the engine running so hard that you can see each individual cylinder head lift as it fires and the tie rods stretch!

Here's some interesting data from one of the makers of these engines, Mitsubishi...the large 12 they make weighs 2030 tons and is 23 meters long and 13 meters high. One model, RTA84T-D, has a top speed of 70 rpm and a ten foot stroke.
http://www.sdia.or.jp/mhikobe-e/prod.../sulineup.html

Ken

[moosehead9]

Gas engines must stay within a certain range of fuel/air ratio in order to run. Spark ignition means the mix must be rich enough but not too rich to be ignited. You can find charts that tell the explosive mix for fuels and if the mix is out of that range it won't light. Gas engines are set for 14.7:1 for the best compromise between power, mileage and emissions. They will run as rich as 12:1 or as lean as 17:1 but the rich mix makes soot and high CO and hydrocarbon emissions. The lean mix will make more NOx emissions and overheat valves and pistons. No fuel burns as a liquid...it must be vaporized to burn. Gas is injected on the intake stroke then compressed this allows it to vaporize. The flash point for gas is around -35F the flash point for
diesel fuel is a minimum of +125F (by federal law). Most is more like +140F or even +160. The diesel fuel is injected near the top of the compression stroke when the chamber is at +1000F or more. The fuel lights immediately from the heat. A small amount of fuel means little power (as in at idle) a lot of fuel means much more power. The only time fuel/air ratio matters with diesel is the max amount of fuel that can be injected with a certain amount of air. That will depend on engine speed and turbo boost. Fuel proprties charts list the mix as being 14.7:1 for diesel (by weight not volume). I know a guy who retired from GM's EMD division and he told me by volume anything richer than 25:1 on diesel would make a lot of smoke. Some engine makers list that spec and usually say 21to23:1. Diesel is backwards from gas...diesel gets hotter when you add more fuel and cooler when fuel is cut back. The only time lean can be a problem for diesel is if the engine as at full temp, full load, full boost, and full throttle...if the fuel is cut off suddenly then it can melt pistons nearly instantly.
That is why all the pulling tractors and truck use an air damper on the intake as an emergency shut off. Whan people mod a diesel high exhaust gas temps are a big problem. Adding more fuel with bigger injectors or remapped ECM's can over fuel and cause engine damage.
Bigger turbos are expensive but usually the only real solution to high egts from over fueling. Most diesel makers say a max of 1350F for
any length of time and 1250 for contiunous duty. If it hits 1500 or more you had better back off, change to a lower gear or if boost is below normal stop and see why it dropped. Most small light or med
duty diesels do have aluminum pistons. Most large heavy duty engines do have steel domes. Nearly all have oil spray cooling on the pistons. The light/med duty engines don't run as much boost and are not tuned for as much power. Modern diesels can make more than two pound feet torque for every cubic inch of displacement. Try that with a gasser! So, unless they are under hard throttle diesels are always leaner then the max allowable fuel/air ratio. All now have fueling based on boost pressure to prevent smoke during start outs and gear changes. Remember when trucks made a cloud off black smoke with every gear change? Now when boost drops the ECM cuts the fuel rate back below the max for that rpm and boost level.
I hope this sheds some light on your question.