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jjuutinen · Joined: 13 Jul 2003 Posts: 180

In Graham´s newest book he wrote that the ideal charge temperature is 70-80 deg C. Now, I believe he means that as far as carburetted engines are concerned (or any engine where the fuel is introduced before the inlet valves) due to fuel vaporization issues. One would believe that in direct injection engines for all practical purposes the lower the induction air temp, the better. Any comments?

rhaus · Joined: 08 Feb 2005 Posts: 20

I would guess you are likely correct except in cases where the air temperature is SO cold that some of your combustion energy has to be expended to heat the air. This is not much of an issue, certainly not in spark ignition engines, but it does become a factor for diesels (which depend on the heat of compression to ignite the fuel. My diesel truck seems to run best when the outside air temperature is between +10 and +40 degrees Farenheit. When the outside air temperature is around -20 to -50 degrees (common in Fairbanks, AK) the truck runs fine but with noticeably less power and a little more bluish smoke due to incomplete combustion I suspect. ---rh---

jjuutinen · Joined: 13 Jul 2003 Posts: 180

Is your truck naturally aspirated for that might explain the problems?

rhaus · Joined: 08 Feb 2005 Posts: 20

Nope, it is turbocharged...but not intercooled....some of the apparent performance loss is no doubt due to the differential/transfer case lubes being like molasses at those temps, but the difference in exhaust color is definitely a combustion issue...even with the engine fully warmed up it is noticeable at the lowest of winter air temperatures--and not just my truck, I see the same from others.

I am not certain where you live, but I have noticed many "unusual" occurances with all types of machinery when the temperature drops into the region of 40 to 50 below. ....go take a hot cup of coffee out on a day like that and toss it in the air...it will burst into ice crystals and float away....I even carried liquified propane around in a bucket once just to show someone I could. (it boils at -41 and we were at -55 that day)...fan belts break because they become too brittle, tires can too (but they usually just get the flat spot from sitting over night--rough ride for the first ten or twenty miles) Perhaps most seriously is the congealing of hydraulic fluids for brakes, clutches, landing gears, firetruck hook-and-ladders, etc.....I've seen all kinds of stuff break because it got cold.

And yes, at those temps, you can leave a contrail down the highway!!

rh

jjuutinen · Joined: 13 Jul 2003 Posts: 180

I live in Finland. We too have sometimes quite cold here. At -30 deg C the tires indeed give a bumpy ride! I just wonder how sensible it is to have a rubber band drive for camshafts in vehicles subject to such temperatures.

rhaus · Joined: 08 Feb 2005 Posts: 20

I agree a hundred and ten percent on that point! I have long argued that belts should not be used on critical items like camshafts...granted, chain drives and gear drives will wear out, but they give more warning before they fail. I've seen too many expensive engines reduced to scrap by failed cog belts on camshafts---sometimes even with low mileages too!

So far I have managed to avoid owning a car with that piece of inferior technology.

We have a Piper Navaho here at work...it uses a belt drive for the alternator...granted, it is a twin so you may still have the use of the second engine's alternator (provided you didn't already loose that engine for some other reason) and they get very frequent inspections...but it still bothers me a bit. Perhaps the scariest use of a belt that I have seen in aviation is to transmit power from the opposed engine to the transmission for a helicopter (a Robinson, I think) ...you know how well helicopters fly when you cease to apply power to the rotors.....that one really bothers me.

rh

dstauffer · Joined: 17 Oct 2005 Posts: 8

I suppose a criterion or criteria of what is best is important.

Low temperature is good for volumetric efficiency, and specific power (horsepower per engine pound).

On the other hand, for thermal efficiency, a higher temperature may well be better, as someone has already noted.

It is not unusual at all for specific power and thermal efficiency to be at odds. For instance, long stroke low rpm engines can frequently generate higher thermal efficiency, but at the expense of heavier engine per unit horsepower (low specific power).

So, in an aircraft context, do we want performance, say as in a fighter, or do we want range, as in a bomber or transport?

jjuutinen · Joined: 13 Jul 2003 Posts: 180

Good points. Let´s assume we have a direct injection SI engine like the DB 605 and we want to get best sfc at e.g. 60% power, say 1.0 ata/1900 rpm. What is the ideal air temp before inlet valves? Assume that outside air temp does not vary with charge temp.

jjuutinen · Joined: 13 Jul 2003 Posts: 180

An aside: how harmful highj rpm is for durability it is said that for heavy duty diesels an engine running at 3000 rpm requires heavy meaintenance at 10,000 hrs while an engine running at (I presume this assumes same cylinder dimensions) 1500 rpm does not require that maintenance until 40,000 hrs is reached.

I have allways disliked high rpm engines. At least partially for their sound that reminds me of a 12 V motor just before it quits after being run on 24 V.

szielinski · Joined: 13 Jul 2003 Posts: 94 Location: Canberra, Australia

Question for rhaus,
Does the blue smoke on cold days reduce if the revs are reduced?
_________________
“Make everything as simple as possible - but not simpler” – Einstein.

rhaus · Joined: 08 Feb 2005 Posts: 20

Nope, no reduction in smoke with rpm drop....if I let the engine warm up a bit, it improves somewhat but will still smoke a bit unless the ambient air temp increases. (especially if you really put your foot into it) At first I thought my truck had a problem, then realized nearly everyone else's did the same under similar conditions.

...another thought, and perhaps a more direct answer to the original subject of this thread: I was reading Graham's new book on the 4360...nicely done by the way!! ...in it I seem to recall a statement about the ideal inlet temp for the engine in the B-36 (if I remember right, it's been a couple of weeks since I read it)....I don't know if that was specific to this engine variant in that specific aircraft or if it could've been applied to others as well....maybe a starting point for you though....seems like it was in the 60-80 degree C range? ....I'll have to go look at it again. Gee, what torture, I have to re-read an engine book!! Smile

rh

jjuutinen · Joined: 13 Jul 2003 Posts: 180

Any idea what´s the temperatute before the inlet valves in your case? Is your engine aftercooled? BTW, this seems to be something not bothering gas turbines in very cold weather... And steam engines (with condensers) also develop more "oomph" in cold weather.

rhaus · Joined: 08 Feb 2005 Posts: 20

I don't know the inlet temp at the valves, maybe I can try to measure it this winter...no, mine is not aftercooled.....as for the steam engines this makes sense because the condenser creates a vacuum that helps "pull" the piston during the exhaust stroke....so a cooler condenser would be better...at least to a point...if it gets cold enough to freeze the water rather than just condense it then all sorts of trouble will result

...as for the gas turbine, this may be because the fuel is fed into the flame front rather than the flame traveling across the fuel...this makes it far less sensitive to temperature and pressure conditions that would wreak havoc on a piston engine.

rh