Strokes.........

[Maccers]

Motorbikes..... Some bikes I am looking at are described as having 4 - stroke engines, then looking at the engine config, they got 1 piston. Hmmmm, i thought that a 4 stroke engine, had 4 pistons, like a regular car engine? Anyone shed any light on this....

 

RM.

[Peter Carmichael]

Suck-Squeeze-Bang-Blow (each stroke is the traverse of the piston from top dead centre to bottom dead centre or vice versa)

 

repeat in accordance with firing order for as many pistons as you have

 

As opposed to 2 stroke which does Squeezebang - Blowsuck.

 

Peterid=blue>

eek.gifSecond loser, Class 5, Curborough May 2002id=red>

[Nick Woods]

4 stroke refers to the number of strokes the engine makes per firing cycle - ie ups and downs of the piston.

 

So for a four stroke engine the cycle is this, starting with the pistonat the top :-

 

INLET - inlet valves open, piston descends

 

COMPRESSION - inlet valves close, piston goes up and compresses the mixture

 

POWER - valves stay closed, spark plug ignites the mixture, burning fuel expands and forces piston doen again

 

EXHAUST - exhaust valves open and the spent gases exit down the exhauist pipe.

 

Thats it really, it works with any number of cylinders from one upwards. It means that you get one power stroke for every two revolutions of the crank.

 

Two-stroke engines work differently, AFAIK they dont have valves inthe cylinder head but have ports in the side of the bores and one upward stoke opens both exhaust and inlet and does the compression at the same time, followed by a power stroke on the way down again. The way the ports are laid out is critical to making the thing work, but even so its possible for fresh mixture to flow in the inlet and straight out of the exhaust

 

 

Nick

P8MRA - The green one with red wings

[Nick Woods]

it looks like Peter & I replied at the same time !

 

 

Nick

P8MRA - The green one with red wings

[James Arnold]

I've read loads of descriptions of what a 4 stroke is over the years, but never a better one than Peter's.

Now to really impress me lets have the same clarity for the 5 stroke cycle (miller cycle I think).

J

 

 

[Super_Rich_Bernie]

Why do cars have 4 stroke engines rather than 2 strokes? Some bikes manage rather well with 2 strokes.

 

If the 2 stroke has twice as many 'bangs' per thousand revs will it not give twice the power?

 

What am I missing? Is it's design inherently less efficient atr getting the gas in and exploding it?

 

Jonathan

 

[oilyhands]

The old Saabs in the 60s had 2 strokes, but they have peaky power delivery, economy and emmissions gradually killed them, the new generation of directly injected 2 strokes are better though.

 

The miller cycle isnt 5 stroke it just has unusual valve timing.

 

Dave

[Nick Woods]

AFAIK two-strokes tend to be thirstier than fours and I believe (although am happy to be corrected on this) that they dont produce as much torque.

 

Having said that I remember Ford trialling some 3-cylinder 2-strokes in Fiestas several years ago. I believe they used overhead valves and some sort of injection system to solve the fuel consumption problems.

 

The early indications were good, they went well and were economical but I dont know what happened to them. ISTR there were other manufacturers also doing the same thing but again I dont know what happened to them.

 

I think it might have been something to do with emissions because 2-strokes use a total loss oil system, ie they dont have a sump like a 4-stroke does but instead special 2-stroke oil oil is mixed with the fuel and gets burnt during the combustion process. This is why you'll often see a blue haze behind badly set up mopeds. I guess all the by-products coming out of the exhaust may have been too hard a problem to crack.

 

 

 

Nick

P8MRA - The greenid=green> one with redid=red> wings

[scooby dooby doo]

a 4 stroke has the big advantage of having oil in the bottom (sump) to lubricate things and petrol in the top (above the piston) to go bang.

A 2 stroke has the combustable mixture above and below the piston so it can go bang above whilst being compressed below. It then comes round the side in a a channel and into the top to go bang etc etc.

So where does the oil come from to lubricate it all? answer is that you feed it fuel with a few % oil in it (1 to 20 mixture i think??). The oil then gets burnt with the petrol. This is bad as oil is designed to lubricate not to go bang - so you get a nasty smokey exhaust. I presume that lubrication is also comprimised so you have to rev slowly (lawnmower) or rebuild often (Race bike). On the plus side you get a bang every rev instead of every other so you can get more power. Not twice as much as its not as good at compressing etc but still a fair bit more.

 

Dave Hooper - North London

dmch2@lineone.net

[cdg]

I have been corresponding for many months with racing drivers in the US who use 2strokes in their sports racing cars. 200bhp per litre is very easily attainable at very low engine speeds. The 1300cc Kohler and AMW engines some of these guys use make about 275hp at 8500rpm, and drive through Hewland transaxles. The 850cc triples (usually snowmobile derived) make around 185-190hp, again at 8500ish rpm. Downsides are the need for regular crank rebuilds, and a power band for racing use of less than 2000rpm, hence the need for the Hewland box. Some intrepid drivers (with the ex snowmobile motors) use the Constantly Variable Transmission from the parent snowmobile, therefore the motor is always kept between maximum torque and max. power. Performance is said to be blistering- for a short while!! Unfortunately belts and motors tend to get VERY hot and reliability suffers.

The larger 1300 powered cars are just "awesome" ...apparently.

 

Chris.

 

ps. 2-strokes can be made to rev at very high engine speeds, and far more reliably than a 4-stroke engine. Single cylinder kart engines generally run to over 20,000rpm. 500cc ADM sidecar GP engines rev to over 12000, and make up to 200bhp. Thats 400bhp per litre !!

 

 

Edited by - cdg on 31 May 2002 01:24:07

[puremalt]

Lets then hear it for ****els. I heard of a seven powered by a rotary once.

Be careful how you say the above.

 

Dave Robertson

[EFA]

What about 3 strokes??? - I used to own one of them!

 

Fat Arn

Visit the K2 RUM siteid=red>

See the Lotus Seven Club 4 Counties Area Website hereid=green>

 

[Geoff Johnson]

Not sure this is much help, or anything else really, but random thoughts go:-

2 strokes - Wartburgs

& at the 3rd stroke - 6 inches

 

Must be having a very boring day at work, still the caterham is outside for when I go home.

 

Geoff

J392PPD

VX & Flares how untrendy can I be?

[Simos]

Useless 2 stroke info you didn't want to know about

 

CDG, I used to (occasionally) get 22,500rpm out of my kart engines on the straight of certain tracks. You're right, a 4 stroke with more moving parts and more importantly moving valves would be almost impossible to get reliable at this pace (BMW F1 = 18/19,000 rpm only ) but as to reliability... seizes were a fact of life and the time between rebuilds for engines on that track was 15-30 mins.

 

The most common failure mode was piston seize in the cylinder due to lack of lubrication - because the lubricating oil is supplied in the fuel, lifting off at the end of the straight when the ring/cylinder interface is red hot effectively cuts off the fuel and hence oil... it makes a noise like an old cash register being opened "chiiiiing" (oh, and it locks you rear wheels and you spin into the scenery at 80mph). If you watch the fast boys they'll 'tap' their hand over the inlet of the airbox just before they lift off at the end of long straights, this is to choke the engine momentarily and make it suck in a large amount of fuel to cool and lube the piston as you lift off and hit the brakes. The second most common reason is leaning the engine at such high revs where the carb is tuned for the more usual lower revs (the corners where you'll make time). This is a learning experience, first day on a new track is always exciting as people find out where it breaks engines...

 

If you don't rebuild (and manage to avoid seizing) the failure mode is piston disintegration as the skirt which has been banging around in the cylinger at stupid revs comes away from the crown and lots and lots of bits of hot alloy come out through the carb and exhaust. This one makes a bang noise and may or may not lock your wheels.

 

Those engines did not even start to come "on port" till 14,000 rpm, fluff a corner and drop the revs to 9,000 and it'd die.

 

For those that are interested (is anyone still reading ?) there are three types of 2 stroke race engine used in karting: ported, rotary and reed. There are quite a few subclasses.

 

If you're still reading...

 

Ported engines use the piston itself to cover up the ports leading to the carb, exhaust pipe and transfer ports from crankcase to cylinder. This the simplest 2 stroke, lowest power per volume and most reliable, entry level kart classes and your strimmer/chainsaw use this engine type.

 

Rotary engines use a flat rotating plate with slots cut in it connected to the crankshaft to open or cover the ports. These engines have the carb on the side of the engine. Because you can vary the port overlap and duration these are very tunable and are the highest power 2 strokes. Super A uses these engines and 35+ hp from 100cc was normal on 32mm slide carbs (probably more now)

 

Reed engines use a set of carbon fibre reeds (like a musical instrument) which are flat flexible plates set in a frame shaped like a "V". The plates are fixed at the open end of the V which is connected to the carb. As the piston goes up and creates the partial vacuum in the crankcase it sucks fuel/air past the reeds which flex and open. On the power stroke as the pressure rises in the crankcase (which pushes fuel/air through the transfer ports into the cylinder) the pressure presses the reed against the frame sealing the gap and stopping air flow back into the carb. These guys have the carb on the front of the engine. This is Intercon A class (which I raced), pretty much the same power as Super A but not as tunable so you can't have differnet engines for different tracks.

 

There, you can forget it all now smile.gif

[Graham Sewell]

The much lamented Orbital engine was a modified two stroke -designed by Sarich in Australian and claimed to be able to produce 100bhp/l with 100miles/gallon and negligable noxious emissions (CO, NOx, etc) without the need for a catalytic converter.

 

Two problems killed it:

Noise, vibration and harshness that a lot of the big car manufacturers were working on.

Fuel mixture ratios were not compatible with catalytic converts (ran too lean if I remember correctly) and Europe and our government dictated cats as the technology rather than low noxious emissions as a result. Therefore, the engine died.

 

Big advantage of 2 strokes is less moving parts and hence simpler engine design. Downside is that it can be wasteful on fuel if you are not careful as the fuel is sucked into the cylinder during the exhaust phase and so could pass straight through.

 

Low tech luddite - xflow and proud!

[scooby dooby doo]

simos - cheers! partly for validating that i managed to not say anything *too* wrong, and partly for the extra info! smile.gif

 

Dave Hooper - North London

dmch2@lineone.net

[cdg]

Simos,

 

Very interesting!

 

I know most of the guys running the big Kohler and AMW engines (flat4 and flat6) use both fuel and oil injection, complete with fully mapped ignition. I think they've got them pretty reliable now, although I would hate to estimate how many motors they blew up getting to that stage.

 

I would never dream of using a 2stroke for circuit racing as I don't wish to spend all of my free time deep in the workshop rebuilding engines- but Speed events?- that's a different matter...!

 

To be continued...

[tony pashley]

A couple of entries (Honda, I think) in one of the Shell Mileage Marathons claimed to use six-stroke engines. I believe the additional two somehow gave an extra compression stroke prior to firing and a scavenging stroke aftewards. Does this make sense?

[Morls]

One of the best comparisons between two and four stroke competition engines has been this years Motorcycle Grand Prix 500s (Sorry, I mean GP1 wink.gif).

 

The 990cc Four stroke Hondas, despite no previous race experience (though surely with a huge amount of testing) have been far superior to the highly developed 500cc 2 stroke conventional machines.

 

Even taking out the "Rossi Factor" (as Ukawa is also doing well), the talk is that next year, unless you're on a 990cc four stroke it's not worth turning up.

 

The reasons cited have been many, but include:

 

Horsepower, not as critical on a bike as in a car IMHO, but handy all the same. The Honda 4 strokes are said to be making 210BHP as opposed to 190bhp from the 500s.

 

Linear power curve, as the resonance of exhaust gasses on a four stoke aren't as critical (because the exhaust valve is there to help trap gasses), the power delivery (for v.high bhp/litre) can be more linear, allowing less dramatic highside potential.

 

The two strokes have theoretical advantages such as:

 

Low reciprocating mass of the engine internals means less gyroscopic precession ( less "straight on-ness when trying to through it on it's side" ).

 

Flattening off of the power quickly after peak, to help limit wheelspin on corner exit.

 

Less engine braking to upset the balance on downshifting.

 

But since it's hard to overtake in corners and easy on straights, the horsepower rich 4-stroke wins over the late braking, faster turning two stoke in a real race.

 

It's a shame really as the NSR500s did sound nicesad.gif

 

In "against the clock" racing, such as speed events or qualifying, the diference is far less, as the nimbler 2 strokes can use their braking and cornering advantage.

 

So... I'll have a 990cc 220bhp V5 four stroke with 6 speed sequential box for the seven then please Soichirosmile.gif And an NSR500 engined Jedi for the hills.

 

 

 

Mark

[cdg]

Mark,

 

The transverse 4cyl.ADM sidecar engine would be a better bet for the Jedi as it's designed for outfits with similar overall weights. Latest versions make 205bhp, but I know where there are several 180bhp earlier ones for sale, c/w RG500 derived 6speed sequential boxes. And don't they sound wonderful?!

 

Chris.