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LargePrime

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Tracy Cook detailed in an article why he put a fuel ratio knob on his EC2 system.

 

http://home.earthlink.net/~rotaryeng/Tracy-art.txt

 

In the section labeled "FADEC - SCHMADEC"

 

Gas ratios are 12.7 - 15-7 to one. Diesel runs 20 - 80 to one. Seems to me the need for mixture control is exaggerated on a diesel.

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IIRC, diesel engines do not have a throttle plate, and control power by setting fuel flow. Airflow is set by RPM, so effectively the "throttle" IS the mixture control, and you can have extremely low fuel/air ratios at low power settings without a restriction in the air intake, which is one of the reasons why diesel engines are more efficient than otto cycle engines.

 

You are correct that in an otto cycle engine with a single power lever (like any car built within the past 20 years) that the computer will set the mixture ratio, but that is not necessary in the diesel engine. It's a lot simpler, and not dependent upon fuel injection for the single power lever.

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Marc

 

You are correct in that there is no throttle plate in diesels.

 

You are correct that in an Otto cycle engine with a single power lever (like any car built within the past 20 years) that the computer will set the mixture ratio, but that is not necessary in the diesel engine. It's a lot simpler, and not dependent upon fuel injection for the single power lever.

 

You suggest a change in fuel ratio's in not necessary in a diesel. Please explain. As I understand it, changing fuel ratios is necessary in both of these engines. That’s how they increase and decrease power from any power setting. This is a detail, but an important one. There is a ratio that diesels will generate max power. With a diesel however there is not a lower limit and an associated heat spike. Ratios of 80:1 are not uncommon, which wont work (no power) in a gas engine. It is this fact that allows no throttle plate, as far as I know.

 

Anyway, the fuel ratio is not magically fixed. Even the old mechanicals ones had a ratio that was increased or decreased with the pedal changes.

 

The question is what will the fuel ratio be at a given RPM? And how will it be set?

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You suggest a change in fuel ratio's in not necessary in a diesel. Please explain.

I worded my response poorly - what I meant was that the COMPUTER CONTROL was unnecessary, since the only thing that was controlled by the throttle WAS the fuel supply, and hence the mixture, with the RPM being dependent upon the load.

 

The question is what will the fuel ratio be at a given RPM? And how will it be set?

It will be set by the load, the throttle setting, and the torque/speed curve of the given engine at a given throttle setting.

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Well, mr largeprime, you like scientific detail, and I like general knowledge, so I'll give you my limited understanding.

 

Diesels are fuel and air injected, a constant charge of preasurized air and a varying charge of fuel. They are happy with a lean mixture, they just don't develope as much power. The power is changed by the fuel charge.

 

They also appear to have a small rpm power range and like to be at a constant rpm setting.

 

This answers everything except a rich mixture, one that the engine/air is not capable of burning. Is that the black smoke we sometimes see or is that just carbon black being ejected?

 

 

Ok so I got it all wrong - someone please correct me

maker wood dust and shavings - foam and fiberglass dust and one day a cozy will pop out, enjoying the build

 

i can be reached at

 

http://www.canardcommunity.com/

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I think I understand. It works the same on gas motors, except gas has a power spike at a lean ratio. If diesel does not have this then there is no reason to lean a diesel. Does a diesel not have a lean burn power point?

 

And the black stuff is both, dust. Unburnt carbon from the diesel which happens in an over rich condition.

 

an aside:

If you happen to have a chance to drive one of the volkswagon TDI's (diesel) you will see why 30%+ of europs new car sales are diesel. No knock, no smell, no smoke.

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Hi I am new in this forum

Being a European I have a different angle to fuel.In Denmark one gallon of 100LL cost $ 4,80 And the Germans fly in from a 100 mile radius to get our cheap gas!!!!

I recognize that wonderful aircrafts are being designed in the US.But the engines....

You probably understand why I am very much hooked on the idea of a diesel in my cozy to come.

The options as I see them are.

The approved 135 hp Thielert diesel I consider to weak allthough it performs a better thrust than the O 320.

Of car engines there could be two alternatives.

The Audi V6 2,5 liter 180 hp is supposed to weigh around 295 lbs.But has a bad reputation regarding reliability.

Volvo has come up with a 2,4 liter 5 cylinder inline developing 163 hp.Like the Audi made of aluminum.I have been mailing with the factory , who politely wishes me all the luck,but are not enthusiastic about supplying data.This engine should be used in Sweden in a PA 25 for towing gliders.

At present time I am truing to get in touch with the owner.

Plane will be called `Hugin`

After Odins raven

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If that Audi is anything like my Audi, that's the dry weight. Well engineered engines, but they are heavy compared to most aircraft engines.

 

Add around #100 lbs cooling systems and oil systems (based on the experience of others with rotary engines)

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With diesels you also must consider torque, not just HP. An example is the Volkswagen diesel which only rates 90hp, but has great acceleration. If I remember correctly, the torque is around 150 ft/lbs. Since torque is a rotational force, its actually a more significant figure to us aviators than HP.

 

Gasoline engines usually have comparable HP and torque figures so someone can just refer to HP to get a general idea of performance.

This ain't rocket surgery!

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Being a European I have a different angle to fuel

I understand that fuel costs are very high in Europe - I used to live there - but have you considered the cost difference between gas and diesel when compared with improving the economy of using gas?

 

For example, it might work out cheaper to run, say a 4 cyl Subaru (like Al Wick) getting 4 - 6 gals / hr on 87 octane, than a big diesel with a higher consumption.

Just a thought.

I can be reached on the "other" forum http://canardaviationforum.dmt.net

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Originally posted by marbleturtle

With diesels you also must consider torque, not just HP....... Since torque is a rotational force, its actually a more significant figure to us aviators than HP.

Popular misconceptions about Torque vs. Horsepower abound. These types of discussions occur on a regular basis on the rec.aviation.<your pick> newsgroups. The statement above is incorrect. Torque IS a rotational force, but HP is the ONLY thing that matters, as long as you have a PSRU to ensure that the propeller turns at the optimal rate. Here are the equations:

 

HP(crank)=Torque(crank) x RPM(crank)

 

RPM(prop) = 1/N x RPM(crank) where "N" is the gear ratio of the PSRU

 

Torque(prop) = N x Torque(crank)

 

The HP and Torque produced by an engine are NOT independent - look at the Torque-RPM curve for any engine at WOT. There is only ONE Torque and ONE HP output at a given RPM.

 

An O-360 produces 350 ft-lb of torque at 2700 RPM, for a power output of 180 HP. At 2200 RPM, it will produce 150 HP and 358 ft-lb of torque.

 

Let's compare this to the Subaru EJ-25 engine, which has a maximum torque of 162 ft-lb, as shown at:

 

http://www.protekperformance.com/rv7/engine/dyno_graph.htm

 

Holy crap - if torque is what we care about, this bites the big one in comparison with the 60 year old Lycoming design - it's less than half the amount of the O-360. However, it puts out 165 HP - almost as much as the O-360 - how can this be? It does it by producing the HP peak at 6000 RPM at 150 ft-lb of torque. You need a PSRU with a ratio of about 2.2, so the RPM at the prop drops to 2700, and the torque at the prop INCREASES to 330 ft-lb - close to the O-360 #'s.

 

So, let's take a look at the Thielert Diesel. It claims 135 HP at 2300 RPM, for a torque of 308 ft-lb. Not as much torque as the O-360, but twice as much as the subaru - wow - a COZY should SCREAM with this puppy, yes? Wait a minute - that's at 2300 RPM, remember? Use a PSRU with a ratio of 0.85 to get the prop RPM back up to 2700 RPM, and the torque at the PROP drops to 262 ft-lb. Less than both the O-360 AND the subaru. This is NOT surprising - the HP is what counts.

 

What "us aviators" care about is power delivered through the prop to the air as a motive force. This comes from the POWER output of the engine, transferred through a PSRU if necessary.

 

There are many books that explain these relationships, and a simple primer is online at:

 

http://homepages.bw.edu/~katchins/csc131common/a_papers/student2/torqueHP.htm

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Originally posted by JakeC

Is it possible to have two different engines that produce the same horsepower at the same RPM and yet produce different torque?

By definition, no. Since:

 

HP(crank)=Torque(crank) x RPM(crank)

 

If you set HP and RPM, then the Torque is defined. Pick any two, there's only one answer for the third.

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Great Post Marc.

dust and I were just having this conversation as I was trying to see how much epoxy I could use in his lay up.

 

I think the torque thing is leftover from cars. Torque you feel in your pants, HP gets you to (way too) fast speeds. This is true because cars dont take much power. Planes do, so the whole thing dont fit in a flying world.

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Holy Crap! Thank you sir! I was about to make the biggest mistake of my life! I had been considering a turbo diesel with 178hp at 2700 RPM. Now I see the error of my ways! The lineal force of HP is so much more important that the rotational force of torque. I can clearly see now that I need to get one of those new non-turbo Civic Si engines with the 182 HP MSL at 8600 RPM with a 3:1 PSRU thingys... just think of the torque multiplication of one of THOSE PUSR... wait, I mean HP multiplier. I don't know why, but for some reason I thought HP wasn't the only engine consideration for "us aviators".

 

Thank GOD you took the time to wander over from your web site to straighten out all us ignorant bastards on this message Forum. I for one will sleep better at night.

This ain't rocket surgery!

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