Category: 

What is the Difference Between a Two Stroke and Four Stroke Engine?

Many motor scooters are made with two-stroke engines.
Spark plugs are found in both two stroke and four stroke engines.
Lawn mowers typically use two stroke engines.
Two stroke engines can allow a small amount of fuel to escape, leading to oil slicks and other pollution concerns.
Jet skis typically rely on a two stroke engine.
Article Details
  • Originally Written By: R. Kayne
  • Revised By: C. Mitchell
  • Edited By: L. S. Wynn
  • Last Modified Date: 08 April 2014
  • Copyright Protected:
    2003-2014
    Conjecture Corporation
  • Print this Article
Free Widgets for your Site/Blog
Artists tend to grow up in wealthier households than doctors.  more...

April 24 ,  2005 :  The world's first cloned dog was born.  more...

The biggest difference between a two stroke and four stroke engine has to do with firing timing, which can often be noticed in terms of sound: the two engine often has a high-pitched, very loud rumble, whereas the four engine tends to have a quieter purr. In most cases this is a facet of basic operation and efficiency. Two stroke engines fire once per revolution, which gives them twice the power of a four stroke that generally fires only every other revolution. Four strokes are more efficient, but they’re heavier and more expensive, too. They’re more commonly found in cars and industrial machinery, whereas things like lawnmowers, jet skis, and most lightweight motorboats rely on the smaller two stroke model. The basic fuel burning and combustion properties of these engines are generally the same, but the differences come with respect to how they convert energy and how efficiently that conversion happens.

Understanding the Strokes

Both types of engines burn fuel through four distinct processes, known as “strokes.” How quickly they complete these strokes is one of the biggest differences, but they both will perform all four at some point.

Ad

Intake is the first stroke. This is when the piston travels down the cylinder while the intake valve is opened to allow a mixture of fuel and air to enter the combustion chamber. Next comes compression. Here the intake valve is closed and the piston travels back up the cylinder, thereby compressing the gasses. Combustion happens when the spark plug ignites the compressed gas, causing it to explode; this forces the piston down. Finally is exhaust, the final step that happens when the piston rises up the cylinder as the exhaust valve is opened, allowing the piston to clear the chamber to start the process over. Each time the piston rises and falls it turns the crankshaft that is responsible for turning the wheels or propelling a non-wheeled vehicle. This is how fuel is converted into forward motion.

When the Engine Fires

In a standard four engine the spark plug only fires once every other revolution in synch with a sophisticated set of mechanisms working to create the four strokes independently. A camshaft must alternately tip a rocker arm attached either to the intake or exhaust valve, for instance, and the rocker arm returns to its closed position via a spring. The valves must be seated properly in the cylinder head to avoid compression leaks, and all of this has to happen simultaneously.

Where the two engine is concerned, by contrast, all four events are integrated into one downward stroke followed by one upward stroke; this is where the name “two stroke” comes from. Intake and exhaust are both integrated into the compression and combustion movement of the piston, eliminating the need for valves.

This is accomplished by an inlet and exhaust port in the wall of the combustion chamber itself. As the piston travels downward from combustion, the exhaust port is exposed, allowing the spent gasses to rush out of the chamber. The downward stroke also creates suction that draws in new air and fuel through an inlet located lower in the chamber. As the piston rises again, it blocks off the inlet and port, compressing the gasses at the top of the chamber. The spark plug fires and the process begins again. Significantly, the engine fires on every revolution in these sorts of engines, which gives them a power advantage at least in the short term.

Weight-to-Power and Orientation Differences

Two engines are usually better suited for settings where quick, sudden bursts of energy are important but aren’t expected to be sustained over a long period. A jet ski, for instance, can often accelerate more rapidly with its two stroke engine than a truck could with a four stroke model, but it also is usually only made to ride for a short amount of time, whereas a truck may go for hundreds of miles or kilometers before resting. Some of this short-term efficiency owes to the two stroke engine’s lower weight-to-power ratio; these engines weigh a lot less on average, and as such can get up and going more quickly. It also takes less energy to propel them forward.

In most cases four engines can only be operated in one direction, too, while there’s more flexibility in engines with just two strokes. A lot of this has to do with the complexity of all the moving pieces, as well as the specifics of the oil sump. The oil sump, which provides lubrication to the engine, is typically only present in four stroke models, and it’s important for helping all the processes happen in differentiated ways. Two stroke engines don’t usually have this, though, which means that they’re able to be operated in almost any orientation with no risk of the oil sloshing out or getting displaced. For things like chainsaws, edgers, and other moving tools, this flexibility is often really important.

Efficiency and Pollution Issues

It's also often true that the smaller, faster engines are more prone to pollution and inefficiency. At the lowest point of travel of the piston when the chamber is filling with fuel and air, the exhaust port often allows some fuel to escape the chamber. This is easily seen with an outboard motorboat, for example; people who look carefully are often able to see a multicolored oil slick surrounding the engine. As a result, these sorts of engines are usually considered more inefficient, and are sometimes also cited for environmental pollution. Although four stroke models are heavier and slower, they usually do a better job of making full use of fuel.

Pricing and Expense

The smaller engines are often less expensive, both in terms of initial purchase price and routine maintenance. They aren’t usually designed to last as long, though. Though there are some exceptions, most can't efficiently run for more than a few hours at a time, and most aren’t designed to have very long life spans, either. Their lack of a dedicated lubricating system also means that they may wear out and break down faster as the moving parts disintegrate.

In part because there is no lubricating system, many two stroke engines also require a special sort of oil, called “premix,” with every tank of gasoline. This adds expense and hassle, and can lead to damage over time if it’s forgotten. In most cases four stroke engines require very little maintenance and upkeep.

Ad

Discuss this Article

anon349976
Post 95

Why do you add the oil in fuel to the 2 stroke engine?

anon223449
Post 90

How one can identify an engine, whether it is a 2 stroke or a 4 stroke engine?

Looking at the cylinder head. 4-strokers have intake and exhaust valves.

anon155763
Post 89

how one can identify an engine, whether it is a 2 stroke or a 4 stroke engine?

anon130315
Post 87

why is a 2 stroke diesel engine used for marine propulsion engines rather than 4 stroke engines?

anon128879
Post 86

The older a 4 stroke motorcycle the more it consumes a lot of gas than a 2 stroke. I have experienced this since I have two kinds of motorcycles, one is a 2T (15 years old) the other is a 4T (8 years old) and the 4T eats more gas than my 2T motorcycle.

And when it comes to lifespan, that's debatable. Here in my country (Philippines) you will see tricycles with a very old 2 stroke motorcycles running smoothly than the newer 4 stroke motorcycles and they live longer than the 4 stroke motorcycles. Hands down everyone.

anon123718
Post 85

4-strokes are better for cars and trucks, proven fact. There have been 2-stroke cars, I've had 2 1960's Saabs that functioned perfectly, 60,000kms before needing the rod bearings checked, oversize SKF rollers were available to uprate the bearings, I've heard of 120,000kms before rings and pistons needed changing, but it did depend on variables- the sport version, especially, was thrashed mercilessly! I loved the Saabs, great winter cars, they loved the cold!

I've raced mx in Sweden during the 70's, the best years, and know for fact that tuned correctly and ridden correctly 2-strokes last very well. I rode both my 250 HVA's for 3 years each, about 25,000 training and practice kms. Both were still on the original crank and bore. Both just improved with age, in fact, perfect piston and ring seal. Hint: Never bore a good cylinder, but try and find a piston a tad bigger than normal if the measured clearance gets too much, a floppy piston can explode. My last piston/cylinder clearances measured ok still. I just changed pistons 4 times a year, 2 sets of rings per piston, and little-end bearings. Racing what to you guys would consider professional racing (but normal to us) I did this more often than a simple club-rider in the USA would need to, it was possible to race an entire season on the same piston (forged Mahle) - and I knew a guy who raced all year on the same ring too! But he was lucky, it was worn so thin it was going to blow any second. And he really DID need a rebore. A double one.

Never use synthetic 2T oils. We ran our bikes on simple 'bought at the gas station' 2-stroke oil for mopeds, and Saab cars (what gas I had left after the race I poured into the Saab tank!) Only run at 20-25:1 ratios. The piston passes heat to the cylinder via the oil, so it needs a good cushion of oil. You can see this if the bore isn't chrome-shiny but slightly wispy-brown. Always jet as rich as possible on a racing-bike, it's about right when the engine stops blubbering about 6-8 minutes into a 30 minute (+2 laps) moto. I never seized or holed a piston, however hot the day, or tough the track. Grunt the engine rather than rev it (use a higher gear and full throttle in the bends, don't open the throttle in or after the bend, open it wide several yards BEFORE the bend. I can guarantee you a 70's HVA will only steer properly if you do this) only use the revs when there are no gears left and you need it -- and I had six gears on the CR250! The only time I ever used 6th, let alone rev-out all it had, was at the GP-track at Huskvarna, the hairiest and fastest track in Sweden, where they tested and developed the HVA's. Scary.

But fun when you got it right.

'Burning oil': Two-strokes shouldn't burn oil. Some of the oil does fly up the transfers and gets burned with the petrol, yes, but this is less than 2-3 percent and is only the lightest fraction of the oil, almost gas-state anyway. A lot of that 2-3 percent is too 'heavy' to actually burn in the incredibly short time available. Most of the oil separates from the petrol in the inlet and crankcase, to coat all the surfaces, to slowly migrate up the transfers into the cylinder (takes about 5 minutes, I'm told, on an mx-engine revving at 6,000rpm). Most of this isn't burned, but, protected by the cool squish (how you can tell if it's right) migrates across the piston to the exhaust port, where some is heated into the carbon you see. But a lot of this oil still manages to migrate into the expansion-chamber. As suggested, some of the oil I've said is immediately carried into the combustion chamber isn't burned, but goes directly out of the port, to condense on the expansion-chamber walls. You can often see dribbles of black oil at the joint at the exhaust port/expansion-chamber, and at the outlet-pipe/silencer. Petroil is a wonderful lubrication system.

Try this: After a hard season's racing grind through the welds on the expansion-chamber and split it. What you can see is very interesting. I've seen engineers writhing with (sexual) pleasure at this sight. The carbon build-up, the shiny, oily bits, tell them better than any mathematics how well the expansion-chamber is designed, how well the engine has been working (and, going by the glares, how fast the works-rider has been riding!)

4-stroke mx'ers: We had them too. I last raced on a Honda XL420 Yoshimura/Bell HVA special. Today's 4-strokes don't turn me on. I wouldn't buy one let alone race one (oh, to be 30 years younger....) The old 4-stokes from the 60's were far better, apart from being very heavy (Sten Lundin Lito weighed 130 kgs, actually quiet light when you consider). They were made from the best materials and, revving to only 6500rpm, could last an entire season before being renovated (valve-springs aside). I can assure you those old bikes were very fast. Just the weight and lack of suspension slowed them, otherwise I'd say they were faster than today's 450 bikes. Today's machines have no flywheels, rev too high, have no punch at the bottom or middle-range at all - a 450 has often less torque than a 250 2-stroke - titanium all over the place, a material that is not suitable for valves at all, for example. They are designed to be thrown-away and replaced after a season. We rode for the love of it, the guys making the European bikes raced for the love of it. Why they went bankrupt.

Some people are just in the business of making money. To hell with the passion, the day they lose money on the bikes they'll get into washing-machines or something.

I'm quite sure that if I were suddenly 30 years younger and on my trusty 6-speed 250 Mag-engined HVA I'd smoke the idiots on their 450's. I'm told by people who still follow me that many of today's riders don't train half as hard as we did. And they are professionals. We were amateurs.

anon107979
Post 84

No whiny two stroke engined motorcycle will ever turn heads like the deep throaty rumble of a Harley-Davidson. Ever!

anon104354
Post 83

shorter life my butt. i have a 64-year-old six-horse that runs better than my uncle's 15-year-old 25-horse. Both are two-stroke.

anon100078
Post 82

Well the two stroke has more get up and go because of the revolution happens all at once so it has more get up and go, as in the four stroke it happens every other revolution witch makes it a little slower.

anon97549
Post 81

which engine has greater fuel economy: 2-stroke or 4-stroke?

If I increase the amount of air which goes inside with same amount of fuel, will the power of engine be increased? Or will it remain same? if it doesn't increase, what's the reason for that?

anon94273
Post 80

#71 Make sure you use 2 stroke only! real gas is bad for your boat.

Tcs882
Post 78

Answers to # 68 and 71.

#68, You can find the 2 stroke gas stations across the street from the 4 stroke gas station, 2 is Red and 4 is Blue, whatever you do, don't buy the one with the Tiger, 6 stroke gas will make your engine take off like a rocket.

#71: The solution to your question is very simple. you sell the 2 stroke and you buy a 4 stroke. see, wasn't that easy? Wish you all a great Memorial Day weekend, remember the Vets and our troops who make it possible for us to have fun on the internet.

anon86545
Post 76

It's great that everyone is interested and trying to learn, but some of the comments on here made me want to bust out laughing.

anon84800
Post 75

A four stroke engine runs off four strokes: intake, compression, power, and exhaust. The four stroke has valves and a cam shaft for exhaust and intake.

A two stroke uses two strokes: the intake after the power and the compression and the exhaust at the same time. Two strokes use two-stroke mix which is often 3:1 4:1 and 5:1 they make mix for this. there isn't a gas station with mix in the gas. you will have to do it yourself.

anon83783
Post 71

How do i replace a 2 stroke with a 4 stroke?

anon83459
Post 70

RPM = revolutions per minute. for the person who wants to know.

anon82479
Post 69

The two stroke smell is not 'unpleasant'!

anon82412
Post 68

What gas station sells two stroke gas and which ones sell four stroke gas. I bought a new boat and really want to take it out, but can't find any two stroke gas stations.

anon80784
Post 67

All I know is my old two stroke Lawn Boy mower lasted four times as long as any four stroke mower I've had since then with practically no maintenance. When it comes to mowers I prefer a no engine push, rotary mower first, followed by a two stroke engine one.

anon78446
Post 65

sorry, I am totally new to this. what is RPM?

anon78302
Post 64

to answer question number 7. No matter what all pistons will never be at bdc. only one or two or three or four, depending if it's a 2 banger 4 banger v6, v8 and basically the pistons that are at bdc don't fire off but the ones and tdc do fire off.

anon78279
Post 63

One comment about all this what they said was wrong about the gas. Gasoline in an internal combustion engine ( basically any gas engine) does not will not and will never explode, gas burns but never explodes. In a compression ignition engine or diesel it explodes, so get your facts right bud.

anon75958
Post 62

OK, so I'm guessing most of the people on here are getting a bike or something bought by their parents and don't know what's going on. A two stroke is a motor that will use more fuel but is a great motor for accelerations.

A two stroke has way more bottom end than a 4 stroke. Why do you think that a 250 2-stroke dirt bike races a 450 4-stroke? The only thing a 4 stroke has got going for it is the lubrication system, and top end power and speed.

You mix gas in a 2-stroke. haven't you been able to read that 4 strokes are the one with the great lubrication system? I'm 17 years old and i know more about motors than some people.

anon74497
Post 61

both have one piston, two does the same in half the time as four, but four does it better! tougher! more power!

anon74496
Post 60

Four stroke: intake stroke, compression stroke, ignition stoke, exhaust stroke

anon74494
Post 59

The starter is what starts the process! Anon's got the idea.

anon73332
Post 57

I'm about to buy a four stroke Apex moped and I just want to make sure I'm making a good choice. any opinions?

anon68879
Post 54

they both just have one piston don't they?

anon66612
Post 53

please give examples of each stroke.

anon64382
Post 50

I would like to know if one was to place a 175hp 4-stroke and a 175hp 2-stroke next to each other on exactly the same boat with same prop size etcetera, what would the difference be in performance? Does one assume that because they are both 175hp that the boats would accelerate at the same pace and have the same top speed?

dmm
Post 49

i am debating as to whether i should buy a 250cc, 2004 Rmz 4 stroke or 2 stroke, i have always rode 4 stroke, but i like the initial speed of the 2 stroke. What is the overall difference in speeds between the two strokes?

anon63828
Post 48

Does a 250cc Suzuki 4 stroke have an overall slower or faster speed than a 2 stroke suzuki? i understand that the start of a 250cc suzuki 2 stroke will be faster off the get go, but overall which is faster?

anon63573
Post 47

i have both a 4 stroke honda and a 2 stroke banshee ..and they both have their benefits. i couldn't pick one or the other.

anon61486
Post 45

2 strokes= mix gas and oil

4 strokes= do not

2 strokes are loud and high pitched

4 strokes are low and sound like there bogging sometimes

2 strokes are better i think, but i have a 4 stroke honda.

anon61109
Post 44

two stroke engines, their performance, pickup and the smell and sound produced by them rocks. i love two stroke bikes especially quads and trails.

anon56417
Post 41

What type of gas can i use in my Honda 150 R Expert then? Its a 4-stroke.

anon54034
Post 40

"what would cause a 4 stroke motor (dirt bike, honda) burn a lot of oil?"

A lot of small 4-stroke engines have what is called a reed valve in the carburetor, sort of acts as a PCV system. Sometimes they stick and cause the piston to suck oil from the crankcase and burn it.

anon45853
Post 38

Hi guys i was just wondering how do you see if the engine is 2 or 4 stroke? by looking at its appearance? does it go with the number of plugs?

anon44224
Post 37

To answer post 29: We don't use the two stroke engine in our everyday use vehicles because most people wouldn't maintain it. Another reason for this would be is that they pollute more.

anon43858
Post 36

"Combustion: The spark plug ignites the compressed gas causing it to explode."

Incorrect. It causes it to ignite and burn at a controlled rate depending upon octane rating.

An explosion in an engine cylinder is called detonation, and will quickly damage an engine.

anon43856
Post 35

Re: What is a Backfire?

"Sometimes referred to as afterfire, backfiring can occur when....."

Incorrect. Two totally different problems. Afterfire is out the exhaust. Backfire is out the intake.

anon43531
Post 33

is a two stroke as powerful as a four stroke?

if so why are we using two stroke in off road vehicles?

anon43324
Post 31

A two stroke engine does not nessarily have less longevity than a 4 stroke. It depends on its design. Catpiller and Kohler make huge two stroke generators that are designed to run forever and incorporate valves just like a 4 stroke engine. It is all in the initial design of the motor itself. Most dirt bike and atv 2 strokes are not designed to run at consistant RPMs whereas marine 2 strokes are. A 2 stroke that is not designed to run at constant RPMs causing hot spots on the cylinder walls which can lead to mechanical failure if the motor is run at a constant RPM. Two strokes have a better power to weight ratio. The engines are small and are more efficient at produding power per combustion cycle. They also produce more pollution and noise and they are not used in automobiles for this reason. A 2 stroke motor will last as long as a 4 stroke if properly maintained and used in the correct fashion, but 4 strokes are more forgiving if you do not properly use them and maintain them, hence they are more widely used since most people do not know how to maintain their equipment.

anon42205
Post 30

is a Honda VT 1800 with 40,000 a good bike or not? What needs to be done to it at this age? Does anyone know the typical age for VT engine before it gets really expensive?

anon41720
Post 29

I get the point of 2 and 4 stroke engine but if 2 stroke engines generate more power, why don't they use them in cars. yes i know that they are loud but couldn't you put a bigger muffler on it to lower the noise?

anon40306
Post 28

see i'm going to get a 250 but the whole 2 and 4 stroke confuses me and i'm afraid if i get one i won't know what to do? what do i do? --

blake44556

anon36349
Post 26

4 stroke engines are more durable, two stoke engines you have to mix gas and oil to certain ratios, some newer scoots do the mixing automatically and you have two tanks, one for gas and on for oil.

get a four. I've seen them with 30000 kilometers with no problems.

anon35896
Post 25

Hey over at the bottom of this article it claims that the 2-stroke exhuast create an unpleasant smell. who says. my lungs were designed to function solely on the gases of a 2 stroke motor. I live off that smell. (I race KTM's, I'm a bit of a die hard 2 stroker)

robert5
Post 23

Thanks anon.

hey mosesport: I think the turning of the motor itself creates the motion.

In cars, there is an electice motor that starts the process.

anon30088
Post 22

You can tell if it is a 2 stroke or 4 stroke by looking at the exhaust. If it is big like real big it is 2 stroke, if it's small and skinny then it's 4 stroke. I know this because I have been riding since I was 4.

anon29343
Post 21

A 4 stoke is different from a 2 stroke because the oil is mixed with the gas unlike the 2 stroke that is now mixed, but what are the other differences?

anon25121
Post 20

i have read the article and i understood the working ,my Q is how can we realize a vehicle after riding it whether it is a 2 stroke or four stroke engine?

anon24916
Post 19

Ya u r right...alternatively there is a bypass arrangement for starting conditions in which some fuel is provided for the start.

anon24017
Post 18

I own 1 2-stroke bike (Banshee) its a 1987, has been very reliable over the years for how rough it has been run, the last topend I had installed lasted for ~2 years +, just about 3 or 4 months ago I went ahead and got a smaller bore set of cylinders.

Two stroke bikes are very fun & reliable if maintained properly, depending on what model bike you have, rebuilding a 2-stroke top end is very cheap compared to rebuilding a 4 stroke top.

2 stroke bikes can make big HP gains a lot easily and cheaply compared to 4 stroke bikes.

anon23717
Post 17

Drifter...the 4 stroke engine in your atv works just like the engine in your car, don't mix the oil and gas

Robert....A carburetor atomizes the liquid gasoline into a vapor making it far more volatile. It also allows control of the engine speed via a throttle butterfly that is connected to the gas pedal.

Waste pump operator.....read the owners manual, you will ruin the engine if you operate it while treating it like an engine type that it is not.

Correction to the text above....Some 4 cycle (stroke) engines have ignition systems that fire the spark plug(s) EVERY time the piston nears top dead center (TDC) but the only on the combustion stroke does the cylinder produce power

anon21910
Post 16

I have a "waste" pump powered with a gas engine. How can i tell if it is a 2 strike or 4 stroke engine?

robert5
Post 15

so what exactly is the purpose of the carburetor in a car?

drifter4
Post 14

we have won a chinese atv.it is a single cylinder, air cooled, 4stroke engine 110cc. Must I add oil in with the gasoline. If so in what amount? I would appreciate any help on this question Thank you Drifter4 out.

anon16228
Post 13

yes 2-stroke motors do have twice as much power compared to the four strokes but even with the correct oil/gas mixture the motor wont last as long. this is why pro motoX riders rebuild there top ends after every few races to prevent them from blowing up. four-stroke motors carry heavy oil in their crank case lubricating the the joints of the piston allowing the motor to last longer. and a 4-stroke honda race motor will burn oil it doesnt mean that the engine is messed up its just due to the high performance motor will eat oil faster i know i ride a crf250r.

anon14391
Post 12

mosesport: When you crank over the starter, the pistons are going up and down... thus "priming" the engine to be started.

joefalcone
Post 11

So... what engine do you have to mix oil & gasoline.

together. 2 or 4 stroke?

anon14301
Post 10

Are you "burning" oil (as in a rich exhaust) or are you losing oil (as in leaking)? Burning oil can be caused by many things including engine wear, particularly if you've been using the wrong oil (too thin), using dirty oil (causes engine wear), or old oil (loses viscosity). If you keep your bike maintained well and it's burning oil, better take it to a mechanic. Could be piston wear or other internal problems causing it.

anon11599
Post 9

what would cause a 4 stroke motor (dirt bike, honda) burn a lot of oil??

anon11077
Post 8

starter turns the flywheel which turns the crank shaft that is attached to rods and pistons, pistons that are within a cylinder, the outcome of the piston going down the cylinder is vacuum, this draws in atmospheric pressure 14.7psi.

anon11007
Post 7

I have read the whole working process of four stroke engine. i have a question in my mind that when first process starts piston came from tdc to bdc. now suppose when we stop the car means switch off the ignition and the piston is in bds than what happen how the intake process starts?

dmj1
Post 4

The 4 stroke engines have a couple more strokes than the 2 stroke ones.

anon4328
Post 3

negative...the starter is a small electrical motor which turns the engine when the key is selected in the "start" position. this is what starts the process--purging the cylinders, drawing in fresh air and fuel, and then the spark, bang, and it's running.

mosesport
Post 2

So I understand the process, but what initially creates the first downward motion for intake when the car is first started? Did I read correctly somewhere else that there is a gas/air mixture left in the piston when the car is turned off, and turning the ignition makes the spark plug go off to start the cycle?

Post your comments

Post Anonymously

Login

username
password
forgot password?

Register

username
password
confirm
email