Difference between 2-Stroke & 4-Stroke Piston Engines

[Grapes]

Anyone? Thanks.

[Flight Safety]

Serious? 2-Stroke engines are rare in aviation I think (except maybe in ultralights).

Mainly, the 2-stroke engine has a 2-stroke cycle, 1 stroke down and 1 stroke up for the piston, then it repeats the cycle. After ignition of the air-fuel mixture with the piston at the top, the pressure drives the piston down thus imparting energy to the crankshaft. At some point not too far from the bottom of the piston's travel, an exhaust port on the side of the cylinder wall is uncovered, which allows the remaining cylinder pressure to expel the spent gases. Farther down near the bottom of the stroke, an intake port in uncovered (usually on the opposite side of the cylinder wall) which allows a crankcase pressurized fresh air-fuel mixture to be released into the cylinder.

As the piston moves back up, the intake port is covered up first. This allows a small portion of the newly blown-in air-fuel mixture to drive out most of the remaining spent gases before the exhaust port is also covered up. From this point on the air-fuel mixture is pressurized, and as the piston nears the top the spark plug fires, igniting the air-fuel mixture. Then it happens all over again.

2-Strokes can either be carburated or fuel injected. The interesting part about 2-Strokes is the way the fresh air-fuel charge (or just air for injected models) is blown into the cylinder through the intake port. Actually the back side of the piston is used to pressurize the charge within the crankcase when the piston moves down. This means that for engines with multiple cylinders, the crankcase has to be compartmentalized with seals on the crankshaft, so that each piston has its own back-side air compression space.

Air taken in through either the carburetor (or throttle body) is feed to the crankcase directly through reed valves that sit between the carburetor and the crankcase. The reed valves allow air to pass through them only one-way, then they close as the crankcase starts to be pressurized by the back side of the piston. The reed valves are very light weight and can open and close at high speeds.

This special use of the crankcase for a 2-stroke engine means that it cannot hold oil in the same way that a 4-stroke engine can. 2-Stokes are oiled either by adding oil to the fuel itself (like in older outboard motors and chain saws) or the oil is injected into the crankcase (newer designs). Either way, the oil is consumed as the engine runs, as there's no way to either store the oil or to recirculate it inside the crankcase.

Whew! Now someone else can take the 4-Stroke engine.

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Safe flying to you...

[Bumblebee]

Great description.....Two-strokes can be pretty efficient too..you've reminded me of my early teenage years messing about with radio controlled 'petrol' engined models.
They actually ran on Methanol with between 5% and 15% Nitromethane added to improve power output and ignition stability. Had an Italian manufactured motor - Rossi- I think, which specified an output of 1.5hp @ 22-24000 rpm from just 3.5cc! This thing powered a 4wd model car weighing about 4kg up to speeds of 65mph+ on short grass....

[Grapes]

Thanks Flight Safety.

Bottom line unlike the 4-strokes, 2-strokes actually have overlapping strokes right?

[twistedenginestarter]

A two stroke is efficient in the sense that it has less moving parts and thus is both cheaper and lighter. Also it potentially outputs twice the power at given revs because it fires twice as often. And another benefit is it's twice as smooth for the same reason.

However you don't even see them on lawn mowers much anymore. It is impossible to get the same degree of fuel efficiency and polution control when you've got incoming fuel getting mixed up with outgoing exhaust. Frankly I wouldn't have believed it could work if I hadn't had a Vespa.

Well actually it didn't work because the plug would foul up after about 5 miles. Not ideal if you've just started a BUZAD 4R SID.

If you are looking for a successor the current piston engine - the diesel is probably the one to put your money on. Your Wankel would seem ideal but it hasn't proved popular.

[gaunty]

Erm
Two stroke engines are much more efficient in turning fuel into noise than four strokes.

[GJB]

Ok, I'll take up the story for the 4-stroke. This is called the 'otto' cycle.

The four strokes are INDUCTION, COMPRESSION, IGNITION & EXHAUST.

INDUCTION:
The fuel/air mix that is drawn into the cylinder, by the inlet valve(s) opening is compressed prior to ignition. The cylinder has travelled downwards, drawing in the fuel/air. The COMPRESSION occurrs by the cylinder travelling up the bore, with both valves closed. By squeezing the gases together they become more volatile. Then follows the IGNITION stroke - the compressed air mix is ignited by the spark plug, forcing the piston down the bore. As the piston travels up once more the exhaust valve(s) opens, and the EXHAUST stroke commences, to expel the by-products of the combustion. Then the process starts all over again, with induction following immediately after the exhaust stroke.

In a 4-stroke engine, each piston in the engine completes these 4 stages.

Hope this helps!


[This message has been edited by GJB (edited 06 March 2001).]

[Genghis the Engineer]

You'll find 2-strokes on a great many microlights, ultralights, and homebuilt light aircraft. They give much better power:weight ratio but burn about 50% more fuel for a given power.

The lack of any valving system, helps the simplicity a lot, as often does air cooling.

The standard 2-stroke you'll find used at present is a Rotax 582/48-99 which burns about 18 litres an hour in the cruise, and at peak power burns 27 litres/hour at about 65sHp. It's liquid cooled, unlike the smaller and older Rotaxes such as the 40hp 447 and the 52hp 503, which are air cooled and thus both noisier and usually more reliable.

G

[gas path]

Grapes
Take a look at http://w3.one.net/~jschust/animations.html it takes a while to load but gives an excellent description of the 2 stroke cycle.

[This message has been edited by gas path (edited 06 March 2001).]

[Grapes]

Jeez, thanks guys,
Great help!

[mik]

One interesting thing about four strokes is that the inlet value opens before the exhaust valve has closed to allow a full fuel/air charge to enter the cylinder.

A racing engine has a big overlap, valves open a long way, and the time when both valves are open is relativly long. Advantages : very high revving (doesn't suffer from charge, i.e. fuel-air starvation), high power.
Disadvantages : very lumpy at low revs, high fuel consumption.

A normal car engine has a small overlap, valves don't open very far, and the time when both valves are open is relatively short.
Advantages : fuel-efficient, will idle smoothly at low revs.
Disadvantages : Suffers from charge starvation at high revs (i.e. "can't breath"), less powerful.

Honda VTi engine : at low revs it has a small overlap with a fairly small valve opening. At high revs a different cam profile comes into play & it behaves more as a racing engine, i.e. you hit the 8700rpm limiter very quickly. :)

[GJB]

MIK

With reference to the Honda V-Tec engine - this is achieved by a variable valve timing mechanism. I don't know what the Honda system is called, but I believe BMW call theirs 'VANOS'.

It is not the cam profile that changes - the lobes are machined and therefore are fixed! It is the timimg of the cam, relative to the 'bottom end' speed that alters. I would imagine that the cam(s) duration is retarded as speed increases.



[This message has been edited by GJB (edited 07 March 2001).]

[mik]

GJB,

Surprisingly the cam profile as seen by the valves DOES change in a Honda VTEC engine.

The way it works is that for example in a VTEC DOHC engine, each pair of valves actually has three cams instead of two, i.e. each cylinder has four valves & six cams.

At low revs (up to ~4500 in a 1.8 VTi), the outer two cams do all the work, directly driving their respective valve rockers. The center, "agressive" cam doesn't do anything except push up on a follower that isn't connected to anything.

At higher RPM, hydraulic pressure (actually engine oil pressure) pushes a metal pin through the valve rockers, locking them to the follower. The valves now must follow the profile of the centre cam. Voila! The valves are now open for longer & open wider.

[GJB]

MIK

Well, I am happy to stand corrected! I haven't seen in the V-TEC but know how good an engine it is.

Impressive stuff and thanks for the explanation. :)

[criticalmass]

Grapes,

4-strokes seem to have better reliability than 2-strokes, and longer TBO times.

I fly an ultralight with a Rotax 2-stroke, the liquid-cooled 582UL which outputs 65hp. It has a nominal overhaul time of 300hrs but with proper maintenance some guys get 650hrs plus before the engine gives up.

Eventually, most 2-strokes just expire with little or no warning, whereas 4-strokes will often plug on for a while whilst telling you they are unhappy. The trick is to land and shut down at least 2 minutes before your 2-stroke expires!! Usually the reverse happens.

An overhaul on a 2-stroke usually costs less than on a 4-stroke because fewer parts are involved. On a Rotax it amounts to virtually replacing every moving part, giving you what is effectively a zero-time unit.

Although I have great confidence in my Rotax, I keep one eye out for potential forced-landing fields every time I fly. In a single-engined aircraft you run out of engines so quickly!

[ft]

GJB, comp-re-what? Oh, you mean the suck-squeeze-bang-blow thing? ;)

Also, I caught an article recently about BMW:s new engine which does not have the choking point of a traditional throttle valve. Instead, the throttle regulates how far the intake valve opens. It was supposed to give quite a reduction in fuel consumption and a few other advantages that I can't remember off the top of my head. If anyone is really curious I can try to find the article.

Hmm, and a quick quiz. If you're looking at the valves in an aircraft engine, how do you know which valve is the intake and which is the outlet - without checking which is connected to the exhaust?

Cheers,
/ft

[ShyTorque]

The inlet valves are usually bigger in diameter and are thinner in cross-section. They sometimes have thinner stems than exhaust valves. They may be of a particular shape to aid gas flow. (Also, they are often marked "inlet" and "exhaust"!!)

Engine inlet air is at lower pressure than ambient as it is being drawn in by the downward action of the piston (unless of course the engine is supercharged). The diameter therefore needs to be as large as possible for good performance through good volumetric efficiency. The incoming gas mixture cools the valve so heat is not a big problem.

Conversely, exhaust gas is at high pressure and extremely hot. The gas flow is not a problem so the valve can be smaller than the inlet. Heat dissipation is however more of a design challenge. What is needed is a lot of surface area on the stem and the valve seat, which are the only paths for heat to dissipate to the cylinder head. The valve is therefore chunkier to deal with heat transfer. Some may be hollow and have sodium inside which liquifies. As this is shaken by the valve motion it helps transfer heat from the head into the stem.

Incidentally, 2 strokes are inherently noisier due to the inlet and exhaust ports being opened and closed by the piston as it travels up and down, at very high speed. Because they open and close extremely quickly, more noise is generated as a characteristic. 4 stroke valves are opened and shut more progressively by the ramps on the camshaft and give a more muted sound.

2 strokes are less fuel efficient and more polluting because they have less direct control over the gas flow, due to the large "valve" overlap required to make them work. 2 stroke diesels are much better as the fuel is directly injected into the cylinder at the correct instant. There is consequently less chance for unburnt mixture to go down the exhaust. Unfortunately the problem of "total loss" of lubricating oil remains, although modern 2 stroke oils do burn more thoroughly as part of the power combustion process.

[IHL]

Flight Safety,

There are 2 stroke engines that have oil in the crank case like a 4 stoke . Detroit Diesel makes several such as the 671 used in trucks and industrial equipment.

IHL

[Shore Guy]

http://howstuffworks.com/two-stroke.htm

http://www.howstuffworks.com/engine.htm

http://www.howstuffworks.com/diesel1.htm

[Flight Safety]

IHL, to my knowledge, 2-strokes require a positive pressure (compared to atmosphere) air/fuel charge to be blown into the cylinder, and this is usually developed in the crankcase. Is this provided by some other means in the diesels you're thinking of?

[Cyclic Hotline]

Don't forget the 2 stroke diesel! Probably the simplest internal combustion ever invented. Few moving parts and no ignition!

They were popular as marine and industrial engines. Plus they could run either forward or backwards, depending which way you cranked them! :)

[helimutt]

I work on a two stroke engine. Just a couple of points about it. Positive air pressure required by two turbo blowers to give up to 2.4 bar scavenge receiver pressure, fed to the cylinders when the ports in the liner are uncovered by movement of the piston. Fuel injected by two fuel valves(injectors)
6 cylinder, reversible, MCR=90rpm, bore 700mm x stroke 2200mm!! 20000bhp.
It's noisy but very reliable. Not much use for aircraft though as it's about 10 metres high.