|Steve G||04/01/2019 10:30:32|
19 forum posts
That' a great question. I have assumed that with only one compression ring, the lubricating oil in the fuel would pass the ring on the cylinder wall and disseminate in the crank case. This would then form a fine mist that would 'wet' all of the roller type bearings in the engine. Note that there are no plain bush type bearings, On the rear of the engine is a breather that emits black oil deposits so that may be indicative of lubrication occurring.
To confirm how well this is accomplishing adequate lubrication I will have to disassemble the engine but I ask those here on the forum whether my theory above is correct or they have a better explanation than I do.
|not done it yet||04/01/2019 11:41:20|
|3757 forum posts|
It’s a 180 degree crank, not the usual 360 parallel twin as fitted to many British m/cycles. It may need to have a higher idling speed because of the unequal firing interval but does away with some of the crank out-of-balance issues of a 360 crank.
More like the Honda 250 and 305 Dreams of the early 1960s and the CB450 Black Bomber later in the decade. The magneto on one of my stationary engines (a V-4) is driven at crankshaft speed, but only revs to about 2200rpm, not over three times faster! The Honda engines simply had two sets of contact breaker points, IIRC - iI’ve not looked at my CB450 for probably ten years.
Regarding lubrication, compression rings are, primarily, just that. Bottom lubed petrol engines utilise oil scraper rings to prevent oil going up the bore, with multiple compression rings to prevent fuel finding its way downstairs, so a necessary added complication.
The two (or typically three) rings in motorcycle (and car) engines may well reduce blow-by to a minimum (I have one engine with about six piston rings, mind - never had it apart and cannot get to the Blackstone manual at present). But I know little re these tiny engines - one and two litre cylinder displacement are more my type of engine!
|Mick B1||04/01/2019 17:13:21|
|1267 forum posts|
I'm obviously talking way above my level here - I never twigged the petroil lube or 180 crankshaft, which presumably are referenced in the drawings or backing literature, or maybe just expected background knowledge. I'm sure I've seen ball and roller mains and bigends in 2- and 4-stroke m/c engines that weren't force-lubricated, so I'd imagine your theory will *probably* work. But of course, in the 2-stroke case, unburnt petroil passes through the crankcase and over the components inside, whereas what's getting there in your engine - if I've understood correctly - is burnt combustion blowby. Will there be any effective oil left in that?
|Tim Stevens||04/01/2019 17:29:21|
1122 forum posts
Mick B1 says: I'm sure I've seen ball and roller mains and bigends in 2- and 4-stroke m/c engines that weren't force-lubricated
Indeed I'm sure you have. The major reason for using roller mains and bid ends is often to avoid the need for an oil pump etc. Especially on two-stroke engines - those which do have oil pumps tend to feed the oil into the inlet stream, and don't have pressure feed (which is difficult in any real sense with a rolling bearing).
Up to the 1930s, many (four-stroke) cars had no pressure feed to the big ends, they relied on a dipper in a trough of oil. The last time I looked, some mowers still did ...
|John Olsen||04/01/2019 20:39:50|
|997 forum posts|
Most four stroke model aircraft engines simply rely on the oil in the fuel to lubricate all of the engine. It might seem a bit sketchy, but remember the fuel has much more oil in it than would be typical for say a modern two stroke motorcycle. A three to 1 or four to 1 mix is pretty usual. So there is plenty of oil around and much of it manages to make its way into the crankcase. Even where big ends are plain bearings they last well.
There is an interesting situation with ball and roller bearings. They either need just a bare minimum of oil, like they get with a traditional two stroke, or, if they are to be fed with oil from a pump, they need a high flow. What you must not do is allow the bearing to be full of oil that cannot escape. The oil will churn and the bearing will overheat. Phil Irving discussed this in his book on motorcycle engineering. So provision should be made for the oil to be able to flow through freely.
The 180 degree crank is awkward from the spark ignition point of view. With the 360 crank you can use a double ended coil and a waste spark on the exhaust stroke for a nice simple setup. Honda did this on the 175/200 twin, which did have a 360 degree crank, and used two of them on the fours. You can't do this on a 180 crank like the 350 had. From a balance point of view there is little to choose either way, the 180 crank gives a rocking couple which can be just as much nuisance as the unbalance from a 360 crank.
The Honda 50 stepthru also used the dipper system for big end lubrication. Larger versions of that motor like the 90 had a pump.
|Mick B1||04/01/2019 21:44:10|
|1267 forum posts|
Thanks for that - all clear now. I didn't know about the high oil content in the mix, I could see there's not really room for wet sump and splash, and I guessed aero-engine revs would (typically) be rather higher than a lawnmower...
|not done it yet||04/01/2019 23:09:15|
|3757 forum posts|
My CD175’s and my brother’s CB160 only had one set of contact breakers, I am sure, and was driven at the usual half crankshaft speed (from the camshaft) but there was no distributor. So it must have had two coils and thus a wasted spark at every break? Amazing that I never gave it a thought in my youth. The bikes just went so well and we rode the socks off them. The 175’s did not handle too well, mind! A mate’s 250 James either went around or inside on the corners, while the Honda misbehaved.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.