I think my wife does the same. What's the difference between a 2 stroke and 4 stroke engine? ... Oh dearest I thought you'd never ask. She's a good wife
My outboard boat motor is a 40hp 2 stroke. It has fewer parts because the intake and fuel are on the first stroke of the engine pulling down- creating a vacuum. The second stroke is upwards where the spark is added to the compression and pushing out the exhaust in the upwards motion pushing it all back down again.. or so I think. 4 strokes have a stroke per motion. Needed more equipment keeping it all on time. So I've heard. I'm a easy 2 stroke man.
I'm sure guest will have a much better explanation..
Ooh, i love this stuff and wish to contribute.
You're essentially correct, 2 strokes can be made with less moving parts but can also need fitted with a valve system similar to a 4 stroke. The real advantage of the 2 stroke is that you get twice the power by engine weight because you're not wasting a stroke, the downside is that it runs hotter and foul easier due to the mixing of exhaust gases in the fuel leading to a less efficient combustion. They're easier to make and can be made smaller and cheaper though.
Lol. Basically all true. To add- the internal combustion engine most widely familiar- like on most cars etc. is a “4 stroke” engine-
Most common internal combustion engines are air pumps that have a shaft in the middle that is spun by whatever is going on inside the engine (there’s lots of types etc..). A 4 stroke engine requires 4 “strokes” of the piston to generate two rotations of the shaft that does the work (turns wheels or a propeller etc..)
1. Intake: air and fuel are “breathed in” to create the conditions for an explosion. Piston draws “back” or “down” to pump air in.
2. Compression: the piston draws “forward” or “up” to compress the mixture and create a dense and uniform mix of air and fuel for a controlled explosion.
3. Ignition or power: in the most common non Diesel engines- a source of ignition such as a spark plug is fired with precise timing to ignite the mixture.
4. Exhaust: the piston and the chamber pressure are used along with some sort of valve to push
the resulting mixture of gases and particulates from the explosion from
The engine thusly creating a state of vacuum in the chamber.
When you have multiple pistons in an engine, they will generally work timed in phase so that the downwards movement of one forces another connected by a lobes or links to travel up, so the pistons are each at opposite ends of the cycle as pairs or in some other combination of phase where uneven numbers exist- though for engineering reasons even numbers are most common with 4 and 8 being particularly mathematically advantaged.
What we can say if two stroke engines in general is that they combine strokes- so the intake stroke and exhaust stroke happen at once. Most 4 stroke engines have the exhaust and intake sides closed for most or all of the opening phase of the other. Two strokes mix the two as one. This allows them to complete all their strokes- an entire power cycle- in a single rotation of the crank shaft. In other words in a shorter length of time the motor performs more work.
Because the design eliminates many of the bulky or complicated mechanical parts required to precisely time the opening and closing of the exhaust and intake valves, 2 stroke engines tend to be smaller, cheaper, and lighter than 4 stroke engines that produce similar work.
It also means that a traditional power rating used for 4 strokes is misleading in a 2 stroke because the 2 stroke produces that power more frequently so can perform work as though it had greater power since it produces more power impulses over a set time.
Now- slightly off topic but my favorite type of engine for personal use is… the “Rotary” engine- aka the Wankel Mazda Rotary Piston engine. The rotary engine is a 4 stroke gasoline (or hydrogen!) internal combustion engine but instead of pistons that move up and down it has rotors- they look like triangles although technically they are 3-flanked Hypotrochoids. But it is a shaft with any number of triangles that spin. This a has some neat advantages- reciprocating piston engine means the piston reverses direction- imagine being shaken back and forth hard. Each time a piston motor turns the crank shaft each piston experiences this but with explosive force at thousands of revolutions a minute. Stress. The rotary spins, in a single direction. It only undergoes change when engine speed increases or decreases.
But while the rotary is a 4 stroke engine- unlike the 4 stroke reciprocating piston which generates 1 power stroke every 2 rotations if the crank, our 2 stroke made shocking power and light weight because it has 1 power stroke for each rotation… the rotary engine turns the crank (it’s called and eccentric shaft..) THREE times each time the rotors spin! So it produces smooth and constant power and is doing 3 times the work over a given period of mechanical movement. But how is that possible if it is a 2 stroke? Well- that’s the neat thing…
Each rotor has 3 faces because it is a triangle. A piston has one face. It can only push or pull air that is in front of that one face. The rotary engines 3 faces mean that it can be completing multiple strokes at the same time. While one fave is breathing in air, another face is already in combustion and another face is expelling exhaust. As it makes a single rotation of the rotor, the face that was just breathing in is now entering compression and combustion as the face that was just in combustion is now pushing through exhaust and the face that was just pushing through exhaust is now drawing in fresh air in the intake stroke. So by the time the rotor makes a single rotation is has completed all 4 phases 3 times.
The most common configuration uses two rotors, a 2 rotor engine has three moving internal parts- the eccentric shaft and 2 rotors. A 1 rotor engine has 2 internal moving parts. And so on.
There is no need for valves or timing mechanisms. The rotors have gear teeth and so unless the actual metal rotors befall a catastrophe- once the engine is assembled it will have perfect mechanical timing for its entire life without any maintenance or external mechanisms. The rotor housings and side irons which sandwich together to make the “engine block” and contain the rotors, have holes machines in them that go straight outside the engine. When the corner of the rotor or the face of the rotor pass over the hole they open or close it and let gas in or out. No valves or moving parts asides the rotors.
This simplicity means that the rotary engine generally can have a very high rom limit compared to similar reciprocating piston engines. A 1.3 liter rotary engine for a production car can easily have an 8,000-10000 RPM ceiling and will reliably operate at that RPM for extended periods without undo stress.
A comparable piston engine might have a 6500 or so RPM limit and long term high RPM operation is very taxing. While it is possible to make piston engines go well into the 10,000+ RPM range these are generally high precision race engines using exotic techniques and materials. A few production cars like the Honda S2000 have achieved this- and smaller piston engines like motorcycles can do it as well- but these engines usually have service lives in the tends of thousands of miles or less vs. 200,000+ for a rotary at that RPM threshold.
It becomes more impressive when you realize that the rotary was doing such feats decades before the modern technology to allow piston engines to catch up existed- though very early rotaries did suffer durability problems that was largely sorted out by the early 1980’s. But at that point the Mazda Wankel rotary was not even 20 years old and only 2 companies on earth had put any serious development into them. Piston engines were over 100 years old and had benefited from 2 world wars and countless huge enterprises and hobbyists pushing many hundreds of billions in development. And even with all that it still would be another several decades before Honda could boast to have made a production engine capable of 100ho per liter.
Mazda became the first Japanese automaker to win the 24 hours of lemans in 1991. They used a rotary powered vehicle which left the runner up- Ferrari- almost an hour behind them across the finish. Rotary engines were subsequently banned and another Japanese brand wouldn’t win Lemans until Toyota just recently managed- and they are the largest or one of the largest auto makers in earth. Rotary engines can often be found powering small piston driven aircraft, and in recent times they have been tapped to be developed as a compact, reliable, efficient generator/range extended for EV as the rotary engine is extremely smooth and reliable with high efficiency when it is kept at a single constant speed and not forced to change speeds.
Chevy- at the time one of the largest names in automotive- had originally intended to make the corvette a rotary engine, the AMC pacer had been intended to be rotary along with quite a few other notable cars. Of all the automakers that bought the pattent for the NSU Wankel engine, only Mazda was able to solve the issues plaguing the early motor- namely that it self destructed. It took them a couple decades to get the formula right for a truly long lasting rotary engine though. Norton made some rotary police motorcycles which were not continued in large part because since a rotary doesn’t reciprocate and sound a single direction, in a motorcycle it generates torque on the roaring axis. In other words the higher you Rev the engine, the harder the bike leans to the direction the motor spins! Not optimal.
You are on point as always, friend. I’d add a few words to the lexicon for the rotary (as a lover and believer in it!) for comparison to its combustion counterpart.
.
Apex seals, and gas consumption.
.
Rotary motors primary issues are apex seals, which need to be replaced very often STILL because of the stress they undergo. The rotor spins and makes contact with the internal walls at certain stages of its rotation. This is at the “apex” or points of the aforementioned triangular faces! These seals make the contact which is what keeps the gases from escaping during intake/comp/outlet during the spin. If these little snots go bad, the motor loses efficacy before failing in some capacity. That can lead to your catastrophes, and it’s not an unusual thing to see.
That’s like… must know if you buy one. Replace your apex seals!!!
.
Secondly: gas! Rotary cars don’t sip gas despite their size and weight. That higher rate of spin and compression it’s so famed for means your fuel is being burnt hotter and faster. The motor has to spin more to make power and maintain idle. Usually they will drink more in comparison to a similar power plant with pistons
A pleasure to meet a fellow rotary lover!
I agree they aren’t the best on gas, though I will say that for a period there they were fairly comparable to piston engines, but as technology started to make more and more efficient piston engines- they were left behind.
We were surprised to find that the late 90’s Subaru 2.5 Impreza which was rated at the same stock HP as the early 90’s non turbo RX-7 got basically the same fuel mileage in the real world! The Subaru however met its death as it chased a non turbo RX-7 through the S’s at the race track and put a rod through its own block while that RX-7 continued to have its $500 rebuilt motor beaten on for many more years.
For the period the rotaries did well on freeway mileage comparatively- with mid to high 20’s in mikes per gallon realistic on a steady feeling distance freeway trip. Once you found yourself in traffic or on the pedal though… well- an acquaintance had a turbo which you could literally watch the fuel gauge drop…
when driving at full throttle. The overall synopsis is they make wonderful race cars but as street cars they aren’t for everyone. lol. When we did endurance racing we did not bring he rotaries. They have the reliability and speed but in races lasting 24 hours or more of driving, the fuel mileage has you pitting too often to be at the top of the podium against competitive teams. So even on the track they do suffer a bit of disadvantage in the fuel consumption arena.
The apex seals I give 50/50. Early cars had poor materials for the seals and the coating on the housings was inferior. Later cars around the 80’s into the 90’s saw continued improvements that increased life span, power, and reliability.
The non turbo cars generally do not have issues with the apex seals unless they are being pushed to extremely high RPM’s over 10,000rpm for prolonged periods. They are a wear part as the apex seals and other rotor seals like corner and side seals are the only part of the rotating assembly that contact the “block” and they seal the chambers to allow the engine to have compression. They perform half the duties of the piston rings in a sense. So they do wear with time or friction, but they can easily last (I’ve owned cars that fit this description) over 200,000 miles on original seals.
The seals can stick- they are held against the housings by springs, the springs can lose tension or more commonly become stuck by carbon. Many common reported problems with street driven rotary come from cars which are not ran hard enough often enough. Rotaries that get babied and seldom see redline often look like crap when taken apart. They need to get hot enough to burn off the carbon deposits before they form but if you get them hot and the engine isn’t running properly (bad air/fuel mixture, poor combustion, excessive oil in the chambers, etc.) you can get worse carbon problems too.
There are a few places apex seals themselves cause problems- but it is ofte n “user” problems. Asides the older model seals which are inferior designs we have:
- bad porting or worn parts. If the engine is ported to make more power and the port size and shape are poor or poorly finished, the apex seal can catch the edge of the port a bit on each rotation- and rotaries do a lot of rotations in a short time! Each time it catches on a port edge or on a burr from poor finishing, it accelerates wear on the seal and they can fail prematurely or catastrophically.
Worn “chrome” on the housing causes two potential problems slog with out of spec irons- the surface is uneven and it’s another way for the seals to catch or load with shocks they aren’t designed for. Then they fail.
In extreme cases- especially turbo cars it seems- the port can be made so large that the seals can literally “fall out” over time. Like spitting a piston ring out your tail pipe!
As mentioned the springs push the seal against the housings so it.. seals. More demanding performance applications can benefit from stiffer springs- or an old trick- doubling up on the springs! But this increased spring tension pushing against the housing generates more friction, more heat, more drag. So the seals face increased wear and potential for failure. The next issue is seal selection itself.
Older cars had 3mm apex seals. Newer cars got 2mm seals. Some engine Builders swear that machining newer rotors for 3mm seals is better for durability. Others contest this, and in non turbo applications especially, where higher RPM are usually seen, even small amounts of weight on the rotor are watched carefully, so high RPM non turn on engine builders have their own thoughts on when it if it is better to use lighter 2mm seals or thicker 3mm seals.
Poor selection there or poor work or issues caused can give problems. Then there are various choices for apex seals. Carbon ceramic seals were some of the earliest and still used “racing” seals. Verylight, but apex seals do not like detonation- when he air fuel mix is ignited at the wrong time. Carbon ceramic seals are strong but brittle andREALLY do not
like detonation. The non turbo rotaries are less prone to detonation and less likely to experience it in mild or stick tune, and they also aren’t under boost and generally aren’t creating as energetic an explosion so the turbos are especially susceptible to apex seal damage from detonation overall.
Turbos also happen to be most peoples choice for making big power and rotary engines, especially turbo ones, can see increases well over 30% to power output over stock from just a well designed exhaust. It is very easy to go off the stock fuel maps or outside the range stock components or tuning can handle.so many people have gone that road- “budget builds” and poor tuning or knowledge etc. and then the motor blows up and they write it off as weak seals or the rotary being fragile. Mazda ran 24 hours making 700hp non turbo and finished over Ferrari. That isn’t fragile.
It just is not as forgiving as your average Toyota or vintage Detroit V8. Rotary engines do not lightly suffer fools unless one just runs a stock NA and don’t try to outsmart the Mazda engines and make 2x the power with a bunch of bolt ons and a limp tune or stock ECU and piggy back at best lol. Near stock non turbo engines practically need someone to try to destroy them just to die.
- other dumb stuff. Mazda assembles rotaries in a clean room and they last long time. Properly spec’d it new replacement parts cost money and time but make a difference. Good build practices are important. Having a good air cleaner- no cleaner because it looks cool or certain (often big name) brand filters and intakes that barely filter…
Rotary engines have ALOT of surface contact in a small space compared to reciprocating piston engines. So if little particles like dust from poor assembly practices or an ineffective filter get inside the engine they tend to stay inside until and unless they are burned up and turn to carbon or tumble and get dragged around until the are spit out. Metal shavings from poor maintenance or wear, flaking chrome broken off through wear, bits of hardened carbon deposits breaking off, impurities in dirty oil or form a poor quality filter… these things are jammed up against the engine and act like sand paper. Most of the ot or parts are sturdy and large enough that it doesn’t really matter except over very long spans of time, but the seals are much smaller and softer and will wear more quickly.
- 3rd generation cars- the “FD” had a 13REW engine with twin sequential turbos. These motors get extra hot, and rotaries already run hot. Certain parts of the emissions system like the air control valve are commonly removed and the twin turbo system is often upset- tempered with or the switch overs are ruined with exhaust and other tuning and then no adjustments are made. The Exhaust has recirculation valve is part of the emissions system but it actually is engineered by Mazda to try and help keep the high internal temperatures in the FD down. Thoughtless modification or removal can cause MORE heat here.
There are some other things there but a combination of its natural design and user choices does make the third generation cars prone to the rotors wearing or basically softening and distorting around the groove in the rotors where the apex seals sit. As these groves pull away or are eroded, the seals have less lateral support and begin to cant in the groove. This makes them more prone to snapping due to mechanical forces acting on the seal as well as combustion chamber pressure, but it also lessens the ability of the springs to push the apex deal against the housings which can lead to poor compression and even detonation which rotaries do not like and is even worse when the apex seal isn’t properly supported because the apex seal grooves are not parallel to the apex seal.
So there are potential issues with apex seals and I do agree that their size, circumstances, and natures do make them more susceptible to forces that can damage them than most of the seals in a piston engine. I can honestly say though that it is often a choice made by a user or builder that ultimately causes the failure. I have not once ever (knock wood) had an apex seal fail on me- I’ve seen plenty of cars that had the issue- but I’ve never had it happen to me or while we were campaigning a rotary vehicle in motorsports. I’ve had oil seals fail and side seals or plugs fail. I’ve had corrosion and weak coolant orings and boost blow the coolant passages in the block (rotor housing) out and leak coolant in to the chamber and turn the engine into a rusty paper weight. I did not have but saw a rotor get jammed into the housing at race speeds and shatter lol.
I’ve seen some interesting failures but I’ve never personally had or while on a team had an apex seal failure. Knock wood.
But rotary engines are rear from perfect. If they had 200 extra years of development by hundreds of countries and governments for totals into the multi billions of dollars I’d be very curious to see how they compared to current piston engines. Alas it has essentially been Mazda and a few specialty privateers and small companies in non automotive fields who have been the sole meaningful contributors to rotary engine develooment since Felix Wankel and NSU stopped.
Oh- last thing I forgot to mention- yes. In general expect fuel mileage to be painful with a rotary, but while they are easily outclassed on MPG by motors which share simillar sizes, they actually do pretty well compared to other non turbo motors of their period which share similar power outputs and/or performance. The rotary also maintains pretty smooth operation which has its benefits. One of their biggest stumbling blocks post emissions has been that current designs require oil be injected into the combustion chamber to decease wear and increase reliability etc. that oil is then burned and creates pollution. As deadlines for bans on new petrol vehicle sales draw closed it becomes more and more likely we ma not see another rotary engine powering a factory new car- a sad thought that hopefully Mazda will make sure doesn’t happen. Hopefully we get af least one. More for the road.
1Reply
deleted
· 2 years ago
*LOL* when I saw this had nearly 40 comments, I reckoned most of them would be by @guest_ - then I saw their 5 word reply early in the thread and thought "hmmm?" and then I scrolled down to find my original assumption to be correct and only THEN I realized what the comments ware actually about. Fully based...
Hell yeah I started the motor discussion. I'm a fan of small slow and steady diesel myself. Rotary engines are a great example of humans thinking outside of the box with a new idea towards a combustion engine not just having a can in a cannon barrel..
@general_failure- I tried and then… I failed so hard.
@typow777- diesels are another awesome engine with so many interesting applications. They can win road races, dominate pulls, run in places and conditions that you’d have trouble keep lost other engines asides maybe a steam boiler working… lol. A very different driving character than the rotary, but another awesome engine.
I'm sure guest will have a much better explanation..
You're essentially correct, 2 strokes can be made with less moving parts but can also need fitted with a valve system similar to a 4 stroke. The real advantage of the 2 stroke is that you get twice the power by engine weight because you're not wasting a stroke, the downside is that it runs hotter and foul easier due to the mixing of exhaust gases in the fuel leading to a less efficient combustion. They're easier to make and can be made smaller and cheaper though.
Most common internal combustion engines are air pumps that have a shaft in the middle that is spun by whatever is going on inside the engine (there’s lots of types etc..). A 4 stroke engine requires 4 “strokes” of the piston to generate two rotations of the shaft that does the work (turns wheels or a propeller etc..)
1. Intake: air and fuel are “breathed in” to create the conditions for an explosion. Piston draws “back” or “down” to pump air in.
2. Compression: the piston draws “forward” or “up” to compress the mixture and create a dense and uniform mix of air and fuel for a controlled explosion.
3. Ignition or power: in the most common non Diesel engines- a source of ignition such as a spark plug is fired with precise timing to ignite the mixture.
4. Exhaust: the piston and the chamber pressure are used along with some sort of valve to push
The engine thusly creating a state of vacuum in the chamber.
When you have multiple pistons in an engine, they will generally work timed in phase so that the downwards movement of one forces another connected by a lobes or links to travel up, so the pistons are each at opposite ends of the cycle as pairs or in some other combination of phase where uneven numbers exist- though for engineering reasons even numbers are most common with 4 and 8 being particularly mathematically advantaged.
Because the design eliminates many of the bulky or complicated mechanical parts required to precisely time the opening and closing of the exhaust and intake valves, 2 stroke engines tend to be smaller, cheaper, and lighter than 4 stroke engines that produce similar work.
It also means that a traditional power rating used for 4 strokes is misleading in a 2 stroke because the 2 stroke produces that power more frequently so can perform work as though it had greater power since it produces more power impulses over a set time.
There is no need for valves or timing mechanisms. The rotors have gear teeth and so unless the actual metal rotors befall a catastrophe- once the engine is assembled it will have perfect mechanical timing for its entire life without any maintenance or external mechanisms. The rotor housings and side irons which sandwich together to make the “engine block” and contain the rotors, have holes machines in them that go straight outside the engine. When the corner of the rotor or the face of the rotor pass over the hole they open or close it and let gas in or out. No valves or moving parts asides the rotors.
A comparable piston engine might have a 6500 or so RPM limit and long term high RPM operation is very taxing. While it is possible to make piston engines go well into the 10,000+ RPM range these are generally high precision race engines using exotic techniques and materials. A few production cars like the Honda S2000 have achieved this- and smaller piston engines like motorcycles can do it as well- but these engines usually have service lives in the tends of thousands of miles or less vs. 200,000+ for a rotary at that RPM threshold.
.
Apex seals, and gas consumption.
.
Rotary motors primary issues are apex seals, which need to be replaced very often STILL because of the stress they undergo. The rotor spins and makes contact with the internal walls at certain stages of its rotation. This is at the “apex” or points of the aforementioned triangular faces! These seals make the contact which is what keeps the gases from escaping during intake/comp/outlet during the spin. If these little snots go bad, the motor loses efficacy before failing in some capacity. That can lead to your catastrophes, and it’s not an unusual thing to see.
.
Secondly: gas! Rotary cars don’t sip gas despite their size and weight. That higher rate of spin and compression it’s so famed for means your fuel is being burnt hotter and faster. The motor has to spin more to make power and maintain idle. Usually they will drink more in comparison to a similar power plant with pistons
I agree they aren’t the best on gas, though I will say that for a period there they were fairly comparable to piston engines, but as technology started to make more and more efficient piston engines- they were left behind.
We were surprised to find that the late 90’s Subaru 2.5 Impreza which was rated at the same stock HP as the early 90’s non turbo RX-7 got basically the same fuel mileage in the real world! The Subaru however met its death as it chased a non turbo RX-7 through the S’s at the race track and put a rod through its own block while that RX-7 continued to have its $500 rebuilt motor beaten on for many more years.
For the period the rotaries did well on freeway mileage comparatively- with mid to high 20’s in mikes per gallon realistic on a steady feeling distance freeway trip. Once you found yourself in traffic or on the pedal though… well- an acquaintance had a turbo which you could literally watch the fuel gauge drop…
The non turbo cars generally do not have issues with the apex seals unless they are being pushed to extremely high RPM’s over 10,000rpm for prolonged periods. They are a wear part as the apex seals and other rotor seals like corner and side seals are the only part of the rotating assembly that contact the “block” and they seal the chambers to allow the engine to have compression. They perform half the duties of the piston rings in a sense. So they do wear with time or friction, but they can easily last (I’ve owned cars that fit this description) over 200,000 miles on original seals.
- bad porting or worn parts. If the engine is ported to make more power and the port size and shape are poor or poorly finished, the apex seal can catch the edge of the port a bit on each rotation- and rotaries do a lot of rotations in a short time! Each time it catches on a port edge or on a burr from poor finishing, it accelerates wear on the seal and they can fail prematurely or catastrophically.
Worn “chrome” on the housing causes two potential problems slog with out of spec irons- the surface is uneven and it’s another way for the seals to catch or load with shocks they aren’t designed for. Then they fail.
In extreme cases- especially turbo cars it seems- the port can be made so large that the seals can literally “fall out” over time. Like spitting a piston ring out your tail pipe!
Springs first.
Older cars had 3mm apex seals. Newer cars got 2mm seals. Some engine Builders swear that machining newer rotors for 3mm seals is better for durability. Others contest this, and in non turbo applications especially, where higher RPM are usually seen, even small amounts of weight on the rotor are watched carefully, so high RPM non turn on engine builders have their own thoughts on when it if it is better to use lighter 2mm seals or thicker 3mm seals.
- other dumb stuff. Mazda assembles rotaries in a clean room and they last long time. Properly spec’d it new replacement parts cost money and time but make a difference. Good build practices are important. Having a good air cleaner- no cleaner because it looks cool or certain (often big name) brand filters and intakes that barely filter…
But rotary engines are rear from perfect. If they had 200 extra years of development by hundreds of countries and governments for totals into the multi billions of dollars I’d be very curious to see how they compared to current piston engines. Alas it has essentially been Mazda and a few specialty privateers and small companies in non automotive fields who have been the sole meaningful contributors to rotary engine develooment since Felix Wankel and NSU stopped.
@typow777- diesels are another awesome engine with so many interesting applications. They can win road races, dominate pulls, run in places and conditions that you’d have trouble keep lost other engines asides maybe a steam boiler working… lol. A very different driving character than the rotary, but another awesome engine.