63 replies to this topic

[TrialsRfun]

Think this topic has gone off the rails a bit after a query about the Sherco came in from another part of the site.
Thank you Howard for the explanation of how the Sherco is oiled, any of us with a long time interest in trials bikes are generally curious about the technology of a new design.
The real question was why do fourstrokes run at a higher temperature than two's when they have only 1/2 the number of power strokes than a twostroke irrespective of the age, design or manufacturer of the engine.
This is something that has puzzled me for a long time so does anyone out there know the answer.

[JTT]

TrialsRfun, on Jul 11 2005, 05:36 PM, said:

This is something that has puzzled me for a long time so does anyone out there know the answer.

Still think it comes down friction, created by all the additional moving parts. Friction is where the vast majority of engine heat comes from anyways. Just have to look at an air compressor to see it in action...no fire...lots of heat.

[Woody]

TrialsRfun, on Jul 11 2005, 05:36 PM, said:

Think this topic has gone off the rails a bit after a query about the Sherco came in from another part of the site.

Yes I know and I don't like the 'hijacking' of topics, but this one was already way off your original question so I thought no harm...

Problem with a question like that though is you're never going to get the correct answer unless it comes from a qualified engine/mechanical/development engineer or whatever their correct title is. Just about everything else is opinion/conjecture. Not unlike opinions on different makes of bike perhaps...

I've always wondered why they run so hot too but have no idea - or is it just an illusion/misconception that they run hotter. Has anyone ever compared temperatures? (serious anorak stuff that...) Plenty of 2-stroke enduro bikes boil up when thrashed through bogs, not just 4-strokes. I know one thing though, I've had both lying on top of me following a spill at enduros and the pipe off either feels just as hot when it's stripping off layers of flesh - or maybe the 4-stroke edges it but the pipe is a fraction of the diameter of the 2-stroke so I guess it would.

And just to answer Webmonkey's question, I bought a 4RT after trying a couple over proper sections and liking it - like you, didn't care what anyone else thought as I'm the one riding it. Just the way it was with the TYZ too...

[Outlaw Dave]

The best way to combat both heat and wear with any engine, is to use the highest quality oil. - and although this may come as a surprise to many people, this is not always a Synthetic.

[charliechitlins]

If there's not pressure-fed oil, what gets it up to the cams?
And what do the journals turn in?
Excuse my ignorance.

[The artist formerly known as Ishy]

what oil pressure does the highly stressed bottom end of the two stroke have

[Howard Wallace]

Honda RS,
How can you say this stuff , again !!!! Think your confused between slipper type ,white metal bearings that require large oil pressure to keep a film between them and the crank journal. And roller bearings, as per the two stroke, that requires a whisp of oil in the fuel to survive.

Sherco put a 4T top on a 2T bottom end ,the oil required for the bottom end is seperate ,and being a roller type bearing requires no oil pressure as per the 2T.

[downunder]

Charlie

Go back to page#2 and read what Howard said about the oil system of the Sherco 4T.

To me it makes sense to build an trials engine as simple as possible. If that means things like oil pumps and fancy electronics can be left for Moto GP and trials focus on lightness and strength that's fine. I'm sure riders who do their own repairs at the SSDT would rather have a set of Allen Keys than a laptop computer.

As far as 4 strokes running a higher engine temp goes, I'm still not really convinced they do, lets face it if it isn't boiling it isn't overheating. XL250 Honda's of the 70's and 80's could run for ever only using minimal cooling fins on bulky and heavy engines. The 2 strokes of the same era used big fins to keep them cool (see photo 1983 360 H7 Montesa). Which engine did run hotter???? Or does the 4 stroke have a greater capacity to operate better at high temprature? If that is the case then the 4 stroke motor could be the best on a hot day????

BJDownunder

ps. At least I know how the 4T gets oil to the top end now, thanks Howard.

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[The artist formerly known as Ishy]

Downunder, have you been taking sneak pictures of Honda RS's service department his new parts bin looks full.

[Rob W]

I have just been searching through my library looking for my four stroke tuning book (by A. Graham Bell) or failing that VMX magazine number 19. Can't find either at the moment. I am sure that the answers to hot/cold 4v2 strokes are in Mr Bells book if anyone has it.

VMX mag 19 has an article about Husky 510 4 stroke enduro bikes with a description of the engine lubrication system. These bikes had a ROTAX engine developed in the '70s. It was a light weight, air-cooled four stroke engine that used a misting lubrication system run by the piston presurising the crankcase. I suspect the 4T uses a very similar set up. ROTAX engines were reliable and powerful. They were sometimes hard to start (tongue had to be held right) and sometimes got a bit hot. Never the less they were very, very durable and were used in many applications from go carts to ride on mowers.

I think the 4T engine uses proven technology and will be a good thing.

Rob W

[AtomAnt]

HondaRS, on Jul 12 2005, 02:03 AM, said:

why no other manufacturer of relatively under stressed four stroke motors, has not adopted the same idea?

You just answered your own question! Oil pressure fed systems are a must for racing engines. Not the case for a Trials bike engine where for most of its time, its hovering around 1500 rpm . Now can you stop going on about the virtues of the 4RT's pressure fed system vs the 4T

I believe that the 4 stroke running hotter is an illusion to a certain degree.
The temperature in the combustion chamber would be similar to other engines.( I will research this) After all, maintaining a consistent temperature will aid the efficiency and performance of the engine and this must be in mind at design stage.
If the barrel was overheating then the water would boil but this doesn’t happen. What you do experience is that the engine in general is hotter on the outside due to more moving parts ,its thermal mass being much more than a 2 stroke and the fact that heat is not dissipated fast enough.( as mentioned above in another post ) a greater surface area to allow for heat dissipation would help (like air cooled fins. ) although air is a bad conductor of heat. Water is much better having the highest thermal conductivity of any liquid.

IMHO

Edited by Webmonkey, 12 July 2005 - 10:24 AM.

[TrialsRfun]

Have always thought the Rotax motors (both 2 & 4 stroke) were top quality, certainly the 280 2 stroke in an SWM that I am familiar with is very well made & there used to be lots of the 4s about in moto-cross so the lubrication system used by them & now adopted by Sherco must be proven reliable & suitable.
Good to know that those who have the bikes are pleased with their performance it can only add to the variety & interest at trials.

[g4321]

Quote

Have always thought the Rotax motors (both 2 & 4 stroke) were top quality, certainly the 280 2 stroke in an SWM that I am familiar with is very well made & there used to be lots of the 4s about in moto-cross so the lubrication system used by them & now adopted by Sherco must be proven reliable & suitable.

There seems to be a misconcepthion that the early Husqvarna fourstrokes used the belt drive Rotax engine & Rotax had a similar oiling system to the Sherco.

The early Husky engine was based on a 2stroke bottom end and was not the same motor that Rotax have been selling for the last 20 years. this started off as air colled and then went to watercooled after a 1 or 2 of seasons.

see picture below for Husky engine.

They were only problematic IF they were not maintained properly.

As for 2 strokes running colder - in theory this is the case in that a fresh charge of cold fuel circulates (is compressed) in the crankcase before secondary compression in the cylinder - hence the bottom end of the engine is cooler than the equivalent 4 stroke where the crankcase is partially full of warm oil.

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Edited by g4321, 12 July 2005 - 09:04 AM.

[feetupfun]

downunder, on Jul 12 2005, 12:13 AM, said:

As far as 4 strokes running a higher engine temp goes, I'm still not really convinced they do, lets face it if it isn't boiling it isn't overheating. XL250 Honda's of the 70's and 80's could run for ever only using minimal cooling fins on bulky and heavy engines. The 2 strokes of the same era used big fins to keep them cool (see photo 1983 360 H7 Montesa). Which engine did run hotter???? Or does the 4 stroke have a greater capacity to operate better at high temprature? If that is the case then the 4 stroke motor could be the best on a hot day????

BJDownunder

Hi BJ downunder.
What you have pointed out about the difference in the finning design of 1970s two and four strokes is interesting but I'm not sure that it means that the two stroke has more problems keeping cool on a hot day (which we certainly get plenty of here down under).

The main reason that 1970s Honda XL250 engines have less finning than air cooled two strokes of the same era is that it is all to do with surface area available for heat transfer. Because of the location of the cam, rocker gear and cam chain case inside the 1970s XL250 Honda engine, the external surface area of the engine available for heat transfer ends up almost large enough even without fins - hence stubby finning. In the case of a two stroke motor, the surface area of the head and cylinder without fins would be way too small. Hence a two stroke of the same power output as the XL250 would need more fin area.

In reality the comparison between these two particular engines is problematic because the H6360 Monty motor would probably be able to produce about twice the power of a standard XL250 so there is a need for additional finning by comparison for that cause as well (waste heat produced increases with increased power output).

Remember that the sizing of the cooling surface area is not only designed to prevent overheating, but also to prevent the engine getting too COLD in service.

[TrialsRfun]

Hard to imagine when I initially posed the question about engine running temperatures that there would be such a response as this, I think for my part at least the question is answered fully and whatever oiling system is provided must surely do the job more thoroughly than the coloured fuel I feed to my 2 strokes.