Winter Layup - 1995 Ducati 900 Supersport

[OddDuck]

Those changes could also be made (explained) by a change in air pressure ...

That's absolutely true. I haven't been measuring all possible variables.

I did a spot of reading after your comment. Yep, atmospheric pressure - and humidity - are big variables as well. The thing is, it's a mechanical carburettor. It doesn't adapt to inlet conditions the same way that ECU and sensor controlled fuel injection could do. About the best that I can hope for is that the slow air circuit can do some compensation for a few millibars up or down.

The AFR gauge lets me know what's happening with the mixture vs external conditions.

Colder weather: mixture leans a point or two
Humidity: mixture gets richer by a point or two
Atmospheric pressure: guesswork, frankly, but less of a change than might be expected. I haven't noticed any discernable changes due to weather, and the bike's never been high enough above sea level for altitude to be a serious problem.

The engine vibration leading to handlebar tingle doesn't seem to associate with the AFR readings, but it does correlate with cold headwinds pretty tightly. Warm day, smooth motor. Cold day, rough motor, and it gets worse at speed.

The thing with the temperature of the fuel bowls... I really do think that something is going on with these running too cold. There have just been too many times it's happened.

So, calculations, starting with latent heat of evaporation, petrol to air. These results are by no means definitive, they guide decisions, nothing more. It's a whole lot easier to spend a night or two running the numbers than it is to build things that don't work. I've made lots of assumptions with these:

1) 100% volumetric efficiency at all RPM
2) throttle position is irrelevant
3) perfect air-fuel ratio
4) air density is 1.2 kg / m3
5) humidity is constant and low enough to not be important

I'm not sure how to insert a table (anyone?) so here's the result for 1000 RPM.

At 1000 RPM, 900cc engine, it's doing 16.7 RPS (revolutions per second). Air volume inhaled per second is 0.015 m3. The mass of air handled is 0.018 kg, with a corresponding 1/15th mass of petrol of 1.2 grammes. These numbers scale directly with RPM, just multiply by whatever number of thou RPM the engine is being revved to.

The heat of evaporation of petrol took some working out and I really hope I got this right... I worked off this wikipedia page: https://en.wikipedia.org/wiki/Petrol...f_vaporization

It's based on relative density, presumably relative to water, and expressed in kcal / kg. This means that in terms of power absorbed I had to link the value calculated, 265.5 kcal / kg @ 20 C, to the mass of fuel being used per second, and I had to convert calories per second to watts.

So:

1 watt = 1 joule / second

1 calorie = 4.185 joules

Again, at 1000 RPM, 1.2 g of fuel needs 0.319 kcal to evaporate, or 319 calories. The heat absorbed doing this is the number of joules per second needed, equivalent to the number of calories / 4.185.

So at 1000 RPM, at 20 C, 76 watts is needed in order to evaporate the fuel.

This number scales directly by RPM so at, say, 5000 RPM, 380 watts is needed. It sounds like a lot of energy but almost all of it will come from the intake air and a warmed-up engine.

What I haven't done yet is to run the numbers again for air at 10 C and at 30 C. I think it's worth doing. Temperature is a variable throughout most of these calculations and those two points should be enough to indicate trends.

[BMWST?]

That's absolutely true. I haven't been measuring all possible variables.

I did a spot of reading after your comment. Yep, atmospheric pressure - and humidity - are big variables as well. The thing is, it's a mechanical carburettor. It doesn't adapt to inlet conditions the same way that ECU and sensor controlled fuel injection could do. About the best that I can hope for is that the slow air circuit can do some compensation for a few millibars up or down.

The AFR gauge lets me know what's happening with the mixture vs external conditions.

Colder weather: mixture leans a point or two
Humidity: mixture gets richer by a point or two
Atmospheric pressure: guesswork, frankly, but less of a change than might be expected. I haven't noticed any discernable changes due to weather, and the bike's never been high enough above sea level for altitude to be a serious problem.

The engine vibration leading to handlebar tingle doesn't seem to associate with the AFR readings, but it does correlate with cold headwinds pretty tightly. Warm day, smooth motor. Cold day, rough motor, and it gets worse at speed.

The thing with the temperature of the fuel bowls... I really do think that something is going on with these running too cold. There have just been too many times it's happened.

So, calculations, starting with latent heat of evaporation, petrol to air. These results are by no means definitive, they guide decisions, nothing more. It's a whole lot easier to spend a night or two running the numbers than it is to build things that don't work. I've made lots of assumptions with these:

1) 100% volumetric efficiency at all RPM
2) throttle position is irrelevant
3) perfect air-fuel ratio
4) air density is 1.2 kg / m3
5) humidity is constant and low enough to not be important

I'm not sure how to insert a table (anyone?) so here's the result for 1000 RPM.

At 1000 RPM, 900cc engine, it's doing 16.7 RPS (revolutions per second). Air volume inhaled per second is 0.015 m3. The mass of air handled is 0.018 kg, with a corresponding 1/15th mass of petrol of 1.2 grammes. These numbers scale directly with RPM, just multiply by whatever number of thou RPM the engine is being revved to.

The heat of evaporation of petrol took some working out and I really hope I got this right... I worked off this wikipedia page: https://en.wikipedia.org/wiki/Petrol...f_vaporization

It's based on relative density, presumably relative to water, and expressed in kcal / kg. This means that in terms of power absorbed I had to link the value calculated, 265.5 kcal / kg @ 20 C, to the mass of fuel being used per second, and I had to convert calories per second to watts.

So:

1 watt = 1 joule / second

1 calorie = 4.185 joules

Again, at 1000 RPM, 1.2 g of fuel needs 0.319 kcal to evaporate, or 319 calories. The heat absorbed doing this is the number of joules per second needed, equivalent to the number of calories / 4.185.

So at 1000 RPM, at 20 C, 76 watts is needed in order to evaporate the fuel.

This number scales directly by RPM so at, say, 5000 RPM, 380 watts is needed. It sounds like a lot of energy but almost all of it will come from the intake air and a warmed-up engine.

What I haven't done yet is to run the numbers again for air at 10 C and at 30 C. I think it's worth doing. Temperature is a variable throughout most of these calculations and those two points should be enough to indicate trends.

there are table icons just below the font icons.The table is just set for the columns and rows required and you just type in the squares

1 column 1 row	2 column 1 row
1 column 2 row	2 column 2 row
1 column 3 row	2 column 3 row

[FJRider]

I haven't noticed any discernable changes due to weather, and the bike's never been high enough above sea level for altitude to be a serious problem.

Talk to racers that do hill climbs ... But air pressure changes are not restricted to altitude changes. ie: Hot and sunny vs cold and foggy.

Air pressure and humidity on any given day can make a huge difference in engine performance ... especially with older style carburetors.

[pete376403]

Featherston is 44 meters above sea level. Your base (garage) is I guess about 10 meters above sea level. Altitude is probably not having as much effect as temperature.

Water cooling looks so much more attractive - you could have water heated manifolds that would be maintained at a more or less constant temperature and then jet accordingly.

[Rhys]

"you could have water heated manifolds that would be maintained at a more or less constant temperature and then jet accordingly"

Ducati had an oil heater for 900-750ss the (factory) carbs for winter use as they iced badly at below 5-10c

[OddDuck]

Talk to racers that do hill climbs ... But air pressure changes are not restricted to altitude changes. ie: Hot and sunny vs cold and foggy.

Air pressure and humidity on any given day can make a huge difference in engine performance ... especially with older style carburetors.

Well... I'm not doubting you. Not at all, altitude (or pressure changes) and humidity are issues. The thing is, realistically there's not a damn thing I can do about them on the road. It isn't impossible to get into the carbs and tweak, but it isn't trivial either.

Shifting needle position: raise tank, remove battery and battery box to access carburettor top covers
Adjusting slow air screw or main air jet: seat off, raise tank, remove pod filters and inlet trumpets (this is about the easiest adjustment)
Change main jet: both side fairings off, seat off so I can raise tank and turn petcock, bowl drain off, change jet and put everything back on again
Change slow fuel jet: as above but take carburettor bowl off too.

Every tweak takes about an hour, give or take, and needs tools and working space. There are bikes out there where carburettor tweaks are relatively trivial - BMW flat twins especially - but this isn't one of them.

Temperature control of the carburettor bowls or even entire carburettor bodies is possible, though, since I have a ready source of wattage available via the electrical system and / or exhaust headers. This could be set up via rider control or automatic operation. Basically I'm trying to work on the things that I can actually change or improve.

It won't be easy to build (at all) but it can be made to adapt to changing conditions, without me having to get in and fiddle with the carburettors yet again... it's been six months of tuning and I'm over it.

Featherston is 44 meters above sea level. Your base (garage) is I guess about 10 meters above sea level. Altitude is probably not having as much effect as temperature.

Water cooling looks so much more attractive - you could have water heated manifolds that would be maintained at a more or less constant temperature and then jet accordingly.

Thanks Pete. I think the manifolds are a big issue on car engines, where there are lots of different path lengths, twists and turns... much less so on this bike. The inlet manifolds are long by bike standards but are really just aluminium pipes with a single 45 degree bend. Provided that they're warm enough from the cylinder head heat, there shouldn't be a problem.

The carburettors are a different story, though, almost none of the engine heat gets to these due to the insulators. They are forward of the cylinders in the slipstream and thus are keyed very tightly to ambient air temperature - there's no air warming from the engine, except in near standstill traffic or a tailwind. Thermal management of these really is 'cross your fingers and hope' engineering.

There's good reason for that. I've been thinking for the last three weeks trying to work out a practical, reliable system for managing carb bowl / body heat. It's not a trivial problem to solve.

"you could have water heated manifolds that would be maintained at a more or less constant temperature and then jet accordingly"

Ducati had an oil heater for 900-750ss the (factor) carbs for winter use as they iced badly at below 5-10c

Thanks Rhys - unfortunately there are two problems with this for what I'm trying to do:
1) the oil heater is for the OEM Mikunis and won't fit straight onto the Keihin FCRs;
2) it isn't controlled, as far as I'm aware. I want to get those bowls to a steady temperature regardless of engine or ambient temperatures.

Carb icing: I think it may be a factor, but I can't be sure without decent measurements.

[Kickaha]

Ducati had an oil heater for 900-750ss the (factory) carbs for winter use as they iced badly at below 5-10c

You had to buy it NZ because although it was known problem they wouldn't do it as a warranty fix

Temperature control of the carburettor bowls or even entire carburettor bodies is possible, though, since I have a ready source of wattage available via the electrical system and / or exhaust headers. This could be set up via rider control or automatic operation. Basically I'm trying to work on the things that I can actually change or improve.

1) the oil heater is for the OEM Mikunis and won't fit straight onto the Keihin FCRs;
2) it isn't controlled, as far as I'm aware. I want to get those bowls to a steady temperature regardless of engine or ambient temperatures.

Carb icing: I think it may be a factor, but I can't be sure without decent measurements.

My brother had a brand new 750SS mid nineties and it suffered the carb icing problem, his fix was to relocate the factory oil cooler to a higher position so the warm air going through the cooler went over the carbs and that was the end of the problem, simple solution

[BMWST?]

could insulating the carbs be part of the fix?

[FJRider]

Temperature control of the carburettor bowls or even entire carburettor bodies is possible, though, since I have a ready source of wattage available via the electrical system and / or exhaust headers. This could be set up via rider control or automatic operation. Basically I'm trying to work on the things that I can actually change or improve.

It won't be easy to build (at all) but it can be made to adapt to changing conditions, without me having to get in and fiddle with the carburettors yet again... it's been six months of tuning and I'm over it.

Is it possible to duct warm(er) air from close to a cylinder (or exhaust) to the carb inlets .. ?? A heat exchanger type setup on each pipe for each carb ... maybe ... ??

[FJRider]

could insulating the carbs be part of the fix?

It's the cold air passing through the carbs that is the issue. Like sitting in an insulated room (mid winter) with all the windows open ... would it be any cooler in the room if it was uninsulated .. ??

[BMWST?]

It's the cold air passing through the carbs that is the issue. Like sitting in an insulated room (mid winter) with all the windows open ... would it be any cooler in the room if it was uninsulated .. ??

yes i see 10 char

[OddDuck]

My brother had a brand new 750SS mid nineties and it suffered the carb icing problem, his fix was to relocate the factory oil cooler to a higher position so the warm air going through the cooler went over the carbs and that was the end of the problem, simple solution

Now that's an idea - it'll get the oil cooler out of the firing line of stones etc too. I like it.

[OddDuck]

Is it possible to duct warm(er) air from close to a cylinder (or exhaust) to the carb inlets .. ?? A heat exchanger type setup on each pipe for each carb ... maybe ... ??

Yeah, that was the original idea until I read Kickaha's post.

It's done a lot in the car world, or was, before injection. The heat exchanger is known as a heat stove, it's about the most basic exchanger imaginable - just a box with an open bottom sitting over the exhaust pipe, with a duct running upwards to an inlet manifold above.

Sometimes there's a butterfly valve activated by a bimetallic spring. Once the spring warms up it closes the valve, the engine now warm and the heat stove not needed any more.

The issue/s I face with the ducati make this solution quite hard to implement - this isn't happening under a hood, there's slipstream. I'd have to insulate the ducts going upward, I'd also have to sort out air flow. Finally I'd have to arrange temperature control via the butterfly valve, with the valve close to the heat stove and the sensor close to the carbies.

It's a whole lot easier to just relocate the oil cooler...

[OddDuck]

Made a discovery today - I think (concerning the issue of carb icing) that the FCR41's have a fairly serious design issue. There's an adaptor connecting carburettor throat to insulating rubbers and then inlet manifolds, this adaptor screws into the carb body.

I've just scanned the thing with the thermal imager, shortly after cold-starting the bike and running for a few minutes. The adaptor is the narrow black band in the image. It's running at between 3 to 5 degrees below ambient, at least, the fragments of PTFE tape on the outside of the thing are. It'll be colder inside of course. It's also a few degrees colder than the carburettor body itself.

This adaptor is precisely where the icing problem is likely to happen - in the immediate fuel-air mixing area just after the venturi, needle and jets. It doesn't have a high thermal mass or a particularly good connection to ambient temperature. There's the thread to the carb bodies, but that's it. Experience has shown that any metal-to-metal contact interface presents a heat flow barrier, i.e. tends to make a thermal insulator. If you want heat to transfer, it's either solid metal or conductive heat sink paste.

I'd assembled this with PTFE tape, wanting both to air-seal the thing in such a way that it could be unscrewed again, and wanting something that the petrol wouldn't slowly dissolve. Unfortunately the PTFE is a pretty good thermal insulator, even over the tiny thicknesses inside a thread.

Right... I'm going to have to either use a thermal epoxy and permanently attach the adaptors, or find a heatsink compound that can tolerate petrol long term. In either case, whatever's used is going to have to tolerate the full engine heat convecting up through the manifolds on shutdowns - the carburettors get quite hot after the bike's parked up. I'll have to properly check what this temperature is.

[OddDuck]

Took the bike for a run into town and back, then had a go at measuring how hot the carbs could get with updraft from a hot engine - most of this will happen due to convection inside inlet manifolds, not rising air around the engine. The pictures say most of it - appears that the carbies reliably don't get hotter than 30 degrees on a winter's day, maybe 50-ish in the middle of summer. I remember touch-testing this much earlier and was quite surprised at how hot the carbs can get after a minute or two parked up.

Quick notes:

Shiny metal is a bad target. The rubber insulating boot was used as a datum because the rubber 'shows' the heat well... the manifold right beside it will be at the same temperature but it's reflecting the surroundings instead of radiating its own heat.

Oxidised cap screw heads are a good target. There's a cavity, which helps, and the oxide layer is a crappy reflector, which means it's a good heat emitter.

The carb bodies are a bit too shiny to really be a good target. I've tried to shoot corners or cavities, rather than flat metal surfaces. The quick test for target surfaces with a thermal imager is to see if they act like a mirror: get something hot at the right angle that its image would bounce to the imager, then see if it shows up. If it doesn't, it's a good surface to read a temperature off.

The screw-in carb body to rubber insulating boot adaptor briefly became hotter than the carb body, demonstrating the thermal separation between the two.