Engine cooling

[Terry Syd]

I decided to start this thread because of a 357 Randy was running that became 'heat soaked'. The power dropped right off, as it usually does when a two-stroke becomes hot. In fact, the difference on a dyno between a 'flash' reading when cold and a hot engine can be a 20% difference in power. That 20% power drop doesn't occur all at once, the power drops off as the engine heat increases. If we limit the maximum heat or even just slow the heat rise we may pick up some useful power.

Two-strokes are to a large degree 'liquid cooled' - the liquid being used is fuel. The extra fuel absorbs heat when it is vaporized. Different fuels have different latent heat of vaporization levels. When using alcohol, the extra fuel and its latent heat of vaporization can give a stock engine another 6% power (increase the compression to take advantage of the increase in octane and you can get a lot more).

However, dumping lots of extra fuel through an engine has its limits, plus it wastes extra fuel.

A chainsaw is an AIR cooled engine. - Even a water-cooled engine is cooled by air through the radiator. The radiator is INCREASING the SURFACE AREA to release the heat and a FAN increases the air flow through the radiator.

Increasing the surface area can only be done to a limited degree on chainsaws, however removing obstructions on the cylinder can increase the area that the air can flow through. Removing casting flash and portions of the cylinder that were necessary for the casting process (but not for operation) can increase the area available to the air flow.

Air flowing onto a cylinder must also flow away from the cylinder in order to have a good air flow. It is not uncommon to find objects on motorcycles that are placed directly behind the cylinder. If the air can't flow away from the cylinder, it is not going to have good air flow across the cylinder.

On all the Huskys that I've modded I block off the 'air injection' system. To make more power, you have to burn more air. That air is sucked in through the air injection system - which comes off the FAN ON THE FLYWHEEL. The more air I burn, the less air I have to flow across the fins. - It is a COMPOUNDING problem the more you mod such an engine, the more heat it produces and the less air you have to cool it.

There is also the issue of engine longevity. Perhaps it never gets hot enough to seize, but a more stable running temperature will give a more stable expansion of the components. If the components are constantly changing tolerances, there will be more wear.

What I'd like to see is some objective evidence of how much extra power, if any, blocking off the air injection system creates. If an engine can loose up to 20%, what if a simple modification could reduce that to 15% - that could be a neat boost in power with minimum effort and probably result in less wear in the engine.

 

[Chainsaw Jim]

I've also blocked off the air injection for that reason. Just be careful where you add holes for intake or the filter will get clogged a lot quicker than expected.

 

[Terry Syd]

What I'd like to see is some definitive testing to see how much can be gained. Perhaps two identical saws tested for extended timed cuts, then one saw with the air injection blocked (stuff a rag in it or something). Then do the same test over again and see the results.

Or maybe someone with an extensively modded saw that is running hot, tested, then with the air injected system blocked off.

or the filter will get clogged a lot quicker than expected

Over here in Oz we get a lot of dust off the hard wood. It gets sucked directly off the bar when cutting. The best way to see that effect is to cut up a blackened stump and then check the flywheel cover and see the traces of black dust that get sucked in the front of the cover. The chips may fly around the air injection system, but the light dust gets sucked into the filter box. - The filter actually stays cleaner when the air is coming in from the back of the saw (and likely cooler air to boot). Perhaps some of those 'fish skin' inlets would help to prevent much of the chips from entering.

 

[Chainsaw Jim]

Over here in Oz we get a lot of dust off the hard wood. It gets sucked directly off the bar when cutting. The best way to see that effect is to cut up a blackened stump and then check the flywheel cover and see the traces of black dust that get sucked in the front of the cover. The chips may fly around the air injection system, but the light dust gets sucked into the filter box. - The filter actually stays cleaner when the air is coming in from the back of the saw (and likely cooler air to boot). Perhaps some of those 'fish skin' inlets would help to prevent much of the chips from entering.

We get the same effect every time we make a cut into fir bark. It's reddish in color and has a stickiness to it. One tank of fuel bucking old growth will make a brand new saw look like it's been rode hard and put up wet.
I compared two 390's..one with breathing holes cut in the back side of the high top cap bypassing the air injection and the other with factory set up. We didn't check temperature differences but what we did notice was the one with holes cut in the back let the filter gather dust much faster than the other. They were both used in the same sawmill on the same log. I'm not trying to contradict what you said, I'm just sharing my findings.

 

[Terry Syd]

Glad you replied as to your findings. They are opposite of what I found in the field, I was surprised at how it reduced the dust on the filter. I had expected the opposite, that is, that is would INCREASE the load on the filter - but it reduced it instead.

Perhaps in your case it is the milling operation, the air flow around the saw, the saw itself - I don't know. We have our experiences and we use what works.

I still would like to see what a reduction in cylinder temperature would do for the power of a saw (leaving aside the extra heat and engine wear). When tuning you start looking for those extra 2% gains. They don't mean much by themselves, but pick up 2% here, another 2% there - after doing that five times you have an extra 10% power. Well, actually a bit more as the gains compound on previous gains.

As far as your filter requirements, from what I can see on the forum, many of the blokes are getting interested in using different filters. I've traveled that route myself and none of my saws have stock filters. I mentioned using an oil soaked 'Chux' kitchen cloth wrapped around the filter. If your filter can accommodate the wrapping, give it a try, I expect you will be surprised at how much dust it will accumulate and still keep breathing.

 

[Chainsaw Jim]

The saws were on their side fastened to a mill without much room for the dust cloud to clear. I'm sure that must be a major factor. I should also add that I never put a screen on the holes I cut and alot of the excess were larger particles.
I'd like to have had better info by making the comparison with a screen.

 

[Terry Syd]

I'm not much into appearances, if it had a high top, I probably would have drilled the crap out of the lid, glued some thin filter foam over the lid to keep the 'Chux' cloth suspended off the holes and then attached the Chux with rubber bands. Then just keep changing the Chux every time it got too ugly.

Practical, but not something you would want to show to your friends.

 

[drf256]

Great thread.

Compression plays a role here and I believe some heat is needed to create combustion efficiency.

After all, energy can't be destroyed, it can only change form.

The more heat you blow off an engine, the more energy you lose. But friction and charge density play a role.

Jim, have you notices an increase in power with alternate intake methods over the air injection system?

I'm having a hard time buying that the system will cause positive pressure in the air box. I can see how it acts somewhat like a prefilter of sorts though.

When I Mod a 262 (all I know) I have a tough time allowing flow to the filter to be limited into the airbox by that small air injection manifold.

 

[Terry Syd]

The more heat you blow off an engine, the more energy you lose.

Well, think about how much energy is 'lost' with a water cooled engine. The water cooled engine makes more power because the charge density is higher. Likewise, if you can get an air-cooled engine running cooler, it will make more power.

 

[drf256]

Well, think about how much energy is 'lost' with a water cooled engine. The water cooled engine makes more power because the charge density is higher. Likewise, if you can get an air-cooled engine running cooler, it will make more power.

Agreed. It confuses me.

They say an iron head will make more power than aluminum because it retains more heat in the chamber. But then, static CR can be increased without detonation and tests usually reveal more power with an aluminum chamber.

You need "enough" heat to make one run at peak performance. Compression increases heat/potential energy in the charge for more complete combustion. It pre-excites the mixture to combust and the spark adds the last bit of energy to complete it. More heat (energy) in the combustion chamber, less energy needed for complete combustion. I believe that is why larger saws like less compression Terry, they can't get rid of heat as fast as a small bore motor.

But now, the extra heat is making the air less dense and you're getting less charge volume. That and higher friction coefficients at higher temps is why less power is being made. Detonation is catastrophic as well.

My point is that it's complicated as hell. Interesting too.

 

[Terry Syd]

Here's a very good link to clarify many of the questions you might have -

http://dragonfly75.com/motorbike/2StrokeDesign.pdf

 

[cus_deluxe]

You need "enough" heat to make one run at peak performance.

this. It would seem that any increase in chamber/cylinder temp beyond this minimum would give diminishing returns. A formula where one could plug in compression, fuel type, chamber size and shape, spark plug type, atmospheric conditions, amount of paint on bar and chain and come up with the ideal temp at which a particular engine should be running.

 

[bmwpowere36m3]

Not apples to apples... but the filter on my CS-590 stays much cleaner and longer than the Stihl 034. The 590 has air injection and being in the northeast, we cut mainly hardwoods. I would not want to give up the AIS for any miniscule amount/if any performance increase to clean the filter more often (lose performance quicker).

 

[bmwpowere36m3]

Air or water cooled engines can make the same power... however water cooled will make the same power longer. Water cooling also allows for different designs, since air flow and size (air cooled heads or much larger).

The more heat you can trap inside the CC the more power you can extract.... but in the real-world, those small differences are minimal. I.e., you need a lot more temp for a small amount of power. Ideally the inside of the CC, piston, cylinder would be made from a material with a low thermal coefficient. Unfortunately combustion engines are just plain inefficient...

 

[Keith Gandy]

Heres an option for the holes cut in the topcover
http://www.frogzskin.com/index.php?main_page=product_info&cPath=34_40&products_id=92

 

[mdavlee]

The air injection is fed with air off the flywheel. Take a filter cover off and rev the saw with your hand over the carb area and you can feel a lot of flow.

 

[Nitroman]

I decided to start this thread because of a 357 Randy was running that became 'heat soaked'. The power dropped right off, as it usually does when a two-stroke becomes hot. In fact, the difference on a dyno between a 'flash' reading when cold and a hot engine can be a 20% difference in power. That 20% power drop doesn't occur all at once, the power drops off as the engine heat increases. If we limit the maximum heat or even just slow the heat rise we may pick up some useful power.

In a super-hot region such as the South Pacific (you), heat would be an issue depending on the season. In cool times not a problem, in the summer, cleaning the powerhead would make sense.

Why don't you take a 394/395 or 3120 jug and have it anodized black. And take another jug to simply paint black with some flat black header paint.
Make a nice long cut and have someone off to the side taking temps on the cylinder during the cut. You'd have to agree where and such, but I think you'd see much more heat radiating off the black anodized cylinder and the painted cylinder versus the clean aluminum.

 

[Chainsaw Jim]

I'd like to know exactly why the power decreases at higher temperature. What is happening to the way the fuel burns to give it less energy. It is timing related?

 

[Keith Gandy]

I'd like to know exactly why the power decreases at higher temperature. What is happening to the way the fuel burns to give it less energy. It is timing related?

Heat = energy doesnt it? Complete combustion is the goal and that takes heat?

 

[Chainsaw Jim]

Heat = energy doesnt it? Complete combustion is the goal and that takes heat?

I'm wondering if the heat can vaporize the fuel at the transfers causing the charge to lose some velocity? I'm also wondering if it renders the sound waves ineffective.