fyrstormer

iTrader: (1)

I've got an old HPI engine that has a piston with a machined hole in one side. I wondered what it was about for a long time, until I examined the block and realized the main transfer port in the sleeve didn't have a corresponding channel in the block. Then I realized the hole in the side of the piston was there to allow the intake charge to travel up through the center of the piston, through the hole in the side, up through the main transfer port, and into the combustion chamber.

So my question is: This engine only seems to run really well when I lean it out to the point that the glowplug temperature is higher than I'm comfortable with. (like, almost 300°F, whereas I usually aim for 230-250°F.) But if the intake charge is passing through the center of the piston on its way into the combustion chamber, then presumably it's cooling the piston in the process. So should I still worry about seeing higher-than-average temperatures at the glowplug on this engine?

Roelof

Those holes are normally for boost or bypass ports. They are there to push the trapped mixture under the piston into the transfer channels to get more mixture where it is needed and remove some pressure from under the piston so it will not be slowed down. Most of the time on the lower half of the sleeve there are 2 matching holes to be found, if not then it will push some mixture directly into the intake port.

fyrstormer

iTrader: (1)

On this engine the hole in the side of the piston lines up with the main transfer port on the sleeve, but as mentioned there is no channel in the side of the block corresponding to the main transfer port. All of the flow through that port comes through the hole in the side of the piston. So...can I be confident that the airflow under the piston is keeping the piston cooler than a comparable engine with a normal transfer port?

Roelof

Because you have a side exhaust engine the oriëntation of the transfer channels could be a bit off. But yes it is possible that it is only at the main transfer port but with the better engines not usual.

http://www.novarossi.it/2012/images/...ande/08723.jpg

On this picture you will see the under edge of the piston is for a part shortened. It has 2 functions:
1) not to touch the crankshaft at the lowest position
2) to open a gap at the intake ports when it is at the highest position.
The long side on the piston is to keep the exhaust closed at the highest position.

But no, it has nothing to do with the running temperature. Thes small simple engines just run on a higher temperature. To give an example, I had a Kyosho .15 engine running at 170 degrees celcius, that hot the blue head colored purple. But hey, the engine did run fine.

fyrstormer

iTrader: (1)

Why are you talking about the piston skirt? That's not what I'm asking about.

Here is what the piston and sleeve look like. You can see the hole in the side of the piston, which aligns with the main transfer port. The piston is oriented correctly inside the sleeve.



Here you can see there is no channel in the block for the main transfer port.





This means all of the airflow through the main transfer port passes through the inside of the piston. So my question is: Does this extra airflow through the piston keep the piston cooler? I'm not asking about the temperature of the entire engine, only the temperature of the piston. All I care about is whether the piston is remaining cool enough that it won't get damaged, even if the temperature of the entire engine is higher than normal.

Roelof

Nope, it has no influence on the operating temperature.

fyrstormer

iTrader: (1)

Then what's the point of a porting arrangement like this?

NitroVein

I would have to disagree, and I believe tests has been done on larger 2-strokes. Getting circulation on the gases bellow the piston crown will cool it, it would be very strange if it didn't. Even a problem on 4-strokes if you don't have enough.

But the reason this engine has a boost port like that is cost. (That's not a bypass port)

fyrstormer

iTrader: (1)

Intuitively it seems to me that it must cool the piston a little. It's a shame there's no way to measure in realtime without a lab full of equipment.

Can it really cost that much less to machine two channels into the block instead of three?

Roelof

The cooling of our nitro fuel is much better than normal gasoline.

I have seen many different port types in same model engines and all were arround the same operating temperature. It is just a design but does not say a thing that a different design is working different. It is all about the designer what he thinks is good.

Maybe the heat is not comming from the normal engine but from the pull starter. The shaft comming out of the rear has no bearing but just a bronze bushing, that will create friction heat (and power loss). And because I have seen many things happen during the devellopment of an engine I know friction heat can raise the temperature a lot.

You have to face it, your engine is just a low cost simple RTR engine. It is cheap made with no special materials and what you will change to it, it will never be as good as an engine from the better brands. The design creates an operating temperature which you should not change when the engine is allready used because after the break-in the fit of piston and sleeve is set to that temperature.

fyrstormer

iTrader: (1)

I understand the fit between the piston and sleeve is dependent on the temperature of the engine during break-in. All I'm really concerned about is whether the piston might be overheating and dragging against the sleeve too much. If extra airflow through the piston helps to cool the piston even a few degrees, then that means the rest of the engine can run hotter without risking damage to the piston.

I'm sure the pullstarter does generate a little heat, but remember the pullstarter bushing is constantly bathed in oil, just like the connecting rod bushings; once the surfaces are polished the friction should be minimal.

I know it will never be a Novarossi engine. I don't expect it to be. I just want to make sure when I run it at a "fun" speed that I'm not damaging it. Since I can't run it again until spring, in the meantime I'm accumulating ideas to improve its performance. I'm also buying different spur gears so I can adjust the gearing in case I can't make the engine run faster. I want this car to reach 50mph, and currently it reaches 47mph but only when the engine runs hot.

Roelof

As long the smoke is there, the sound is good and the performance is OK, then the temperature is also set.

banana rt

it will increase its peak rpm's, just my 2 cents

fyrstormer

iTrader: (1)

I eventually got it sorted out.

http://www.rctech.net/forum/onroad-n...-12r-ss-5.html