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4-stroke cycle

Suck Squish Bang Blow part 2- The Four Stroke Cycle

By Mike Kojima

Now that you know what all of the basic parts of an engine are and what they do, it’s time to understand how they work together in a system.  Almost all cars except for the rotary powered RX-8 and nearly all motorcycles use 4 stroke engines and what is called by engineers, the Otto Cycle to turn the chemical energy of gasoline into power.  Manipulation of the 4 stroke cycle is essential for obtaining more power from a motor so it is important to know what the different parts of the cycle are and how they affect your power output.  Let’s jump into and explain the 4-Stroke Cycle.  Sorry if this is all a bit basic for you advanced people but in order to get into more engine theory later, we have to start with the most basic of basics.

To read part one of this series click here!

Otto
This is Otto, a German dude who was one of the pioneers of the four stroke cycle around the turn of the century.  Otto was probably more of the manager and ideas guy of the group consisting of Otto, Daimler and Maybach who invented the four stroke.  I don't think that Otto was the engineering brains behind the concept but the four stroke cycle bears his name.

 

The Intake Stroke

Intake stroke
The intake or suck stroke.

This is where the suck part of our title comes from because that’s what the engine is doing. Let’s start with the piston at TDC or Top Dead Center.  The intake valve is starting to open by the camshaft as the exhaust valve is closing.  As the crankshaft turns, the connecting rod starts to pull the piston down away from TDC.  The turning crank is linked to the camshaft by a chain or belt so as the crank turns, the intake valve is opened more and more until it is fully open and the exhaust valve is fully closed.  The downward traveling piston creates suction in the cylinder so air and injected gasoline from the intake manifold are drawn into the cylinder by this suction.  This continues until the piston is all the way to the bottom of the cranks stroke or BDC also known as Bottom Dead Center.  Because of the shape of the cam, the intake valve is almost totally closed by the time the piston is at BDC.  At the end of the intake stroke we are left with a cylinder full of fresh fuel air mixture with a closed exhaust valve and a rapidly closing intake valve.

The Compression Stroke

compression stroke
The compression or squish stroke.

 

This is where we got the squish in our title. Now the piston starts its trip upward being pushed up by the crankshaft and the connecting rod.  Now the intake and exhaust valves are fully closed and as the piston is forced upward, the fuel air mix is compressed.  This compression forces the fuel and air molecules closer and closer together until they become a highly reactive explosive mixture, the closer the proximity of the molecules; the easier it is to initiate an explosion.  When the piston nears TDC the ignition system fires the sparkplug which triggers an explosion in the cylinder.

The Power Stroke

the power stroke
The Power or Bang stroke.

You guessed it, this is the big bang. By the time the piston is at TDC the explosion of fuel and air in the tightly contained cylinder is well under way.  The heat and pressure of the explosion of gasoline and air rise rapidly and the piston is pushed strongly back down the cylinder with great force.  This is the driving power that spins your wheels and propels you down the track.  As the piston is pushed down the bore the cylinder pressure starts to decrease as the volume of the cylinder increases.  As the piston nears the bottom of the bore the camshaft starts to open the exhaust valve.
 

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Comments
Fuergrissa
Fuergrissalink
Thursday, March 18, 2010 10:25 PM
I wanted to read the article, but I can't stop staring at that VG. What pullys are you using? It's tough to find a quality set of lightweight pullys for the VG without supporting a certain company known for their beijing xerox business model. Those red ones look nice and I don't reconize them. care to share?
Fly'n_Z
Fly'n_Zlink
Friday, March 19, 2010 4:58 AM
@ Fuergrissa: Mike will correct me if I'm wrong here but I'm pretty certain that those are the Unorthodox Racing pulleys. Unorthodox was on the market way before the rest with a full pulley set for our VG30DE(TT)s.
Fuergrissa
Fuergrissalink
Friday, March 19, 2010 6:22 AM
Thanks, It looks like your right. I recognized the UR crank pulley. I think I didn't consider they were all UR because most people shy away from the stock diameter water pump pulley with an under driven crank pully and I don't believe UR makes an OD water pump pulley
Mike Kojima
Mike Kojimalink
Friday, March 19, 2010 8:31 AM
They are UR pulleys but the crank pulley is not UR but a Fisher active Harmonic balancer.
RCVD
RCVDlink
Friday, March 19, 2010 8:54 AM
Mike, I believe that your statement “…4 stroke engines or what is called by engineers, the otto cycle…” is false. From what I remember otto cycle and a 4 stroke ICE are different. They are similar but definitely different. The otto cycle only exists in theory. As far as I know, no one has been able to reproduce it in real life. In an otto cycle there is no “exhaust” or “intake” stage, which also why the p-v on page 2 is not a p-v of the otto cycle. The otto cycle would have a loosely similar shape but basically has the exhaust and intake cutoff that diagram. I’ll double check my text when I get home; or maybe someone else can support me or prove me wrong, but I’m 99% sure what I have said is correct.
aj_gilbs
aj_gilbslink
Friday, March 19, 2010 11:22 AM
"Otto cycle is the ideal cycle for spark-ignition reciprocating engines. It is named after Nikolaus A. Otto, who built a successful four-stroke engine in 1876 in Germany using the cycle proposed by Frenchman Beau de Rochas in 1862. The ideal Otto cycle, which closely resembles the actual operating conditions, utilizes the air-standard assumptions.
It consists of four internally reversible processes:
1-2 Isentropic compression,
2-3 Constant volume heat addition,
3-4 Isentropic expansion,
4-1 Constant volume heat rejection."

Straight out of -Thermodynamics: An Engineering Approach 5th Ed.

The otto cycle is the cycle for any spark-ignition cycle, so a 4-stroke engine is a form of the otto cycle. Diesels are a form of 4-stroke but use a compression-ignition cycle, and so its is a different cycle, the Diesel Cycle.
aj_gilbs
aj_gilbslink
Friday, March 19, 2010 12:00 PM
Wait hold on after reading that again I realize RCVD is right. They are similar but the otto cycle has two stages of constant volume whereas in an engine the volume changes to be able to add the "heat" or energy neccesary for the cycle and then get rid of it.
Sorry thermo was a year ago and thermo 2 isn't until the fall.
Mike Kojima
Mike Kojimalink
Friday, March 19, 2010 12:58 PM
Are you sure? I thought that the 4-stroke cycle was named the Otto cycle because he was the first to use the 4 stroke engine in a automobile.

Otto was not an engineer or scientist so the thermodynamic study work could not have been done by him.

I don't have the time to pull out my textbooks so what can you guys come up with. If you could append your sources, it would be helpful too.
aj_gilbs
aj_gilbslink
Friday, March 19, 2010 5:40 PM
Here you go Mike, I found a perfect picture showing the difference between the ideal and actual Otto cycle of an engine. Also in my 1st post it says that it was invented by Beau de Rochas, but Otto was the first to build a working engine.
This picture is from:

Thermodynamics: An Engineering Approach 5th Ed.
by Yunus Cengel & Michael Boles

http://www.facebook.com/photo.php?pid=32842103&l=21d4b99852&id=29007866
Mike Kojima
Mike Kojimalink
Friday, March 19, 2010 7:31 PM
I was more interested in information supporting that the 4-stroke cycle and Otto cycle are not the same which I think is incorrect. RCVD usually has something interesting to say so I wanted to see his sources.

I also think historically Otto was not the one who actually built and conceived the 4-cycle engine, it was Daimler and Maybach. Otto was the organization guy of the trio.
aj_gilbs
aj_gilbslink
Friday, March 19, 2010 8:03 PM
Oh I got ya. I think I see what he was getting at though with the difference in the change in volume but I'll let him explain what he meant.

Damn thermo books misleading me.
Mike Kojima
Mike Kojimalink
Friday, March 19, 2010 8:11 PM
As far as I know, the 4 stroke cycle is the otto cycle with stuff showing the intake and exhaust flows, hence the roundedness at the ends of the cycle.
RCVD
RCVDlink
Saturday, March 20, 2010 12:34 AM
My source is same as aj_gilbs except I have a different edition but the information/diagram he quoted is exactly the same. But the main difference I was originally referring to was that there isn’t really an “intake/suck” or “exhaust/blow” step in the otto cycle. Otto cycle is a closed system, ie. theres no mass flowing in or out. Another thing to note is that both otto and 4stroke ICE have 4 steps but there not the same. In the otto cycle, the only thing that happens in the 2nd step is heat is being added to the system. The piston does not move at all in the 2nd step. In the 3rd step you have your expansion. Steps 2 and 3 is basically the bang stage in the 4stroke ICE. So because of that, the piston in an otto cycle only strokes 3 per cycle. The otto cycle is also an ideal cycle (non existant in the real world) so it’s a reversible process. Also as aj_gilbs mentioned you have constant specific heats on the 2nd and 4th step. Heat is introduced and rejected from the otto cycle on the backs of magical unicorns. And finally a mineor difference is the otto uses pure air as its working fluid. So those are the reasons that I think they are different. As far as the history goes, I have no idea. Nor do I care b/c I think the otto cycle is stupid, just like pants. I understand their purpose but I still think it’s stupid. IMO
RCVD
RCVDlink
Saturday, March 20, 2010 12:59 AM
^^edit*...otto cycle only 2 strokes per cycle.
AlexSpecV
AlexSpecVlink
Saturday, March 20, 2010 3:10 PM
lol, "heat is introduced and rejected from the otto cycle on the backs of magical unicorns". Brilliant. Though I think thats not the kind of source Mike was looking for heh. Very familiar with suck, squeeze, bang, blow even read the first renditions of some of this in Sport Compact. Though the Otto cycle is all new to me, more to learn. Good stuff.
RCVD
RCVDlink
Sunday, March 21, 2010 9:45 AM
another correction in that post. i said "...you have constant specific heats on the.." i meant constant specific volume. I appologize, i should learn to proof read.

Anothing difference i was thinking about was that i don't think there is "combustion" in an otto cycle. I'm not sure of the true defintion of the word; but if heating air is not considered combustion then no combustion takes place in an otto cycle. So if thats true then just the wording itself can prove that their different. A 4 stroke internal combustion cycle is not the same as a cycle that has 2 strokes and no combustion.

Alex, i meant to imply that the heat is brought in and out of the system by nothing. The otto cycle is an ideal cycle so there is no component that brings the heat in and out. It just does.



Mike Kojima
Mike Kojimalink
Sunday, March 21, 2010 12:24 PM
I briefly looked over my books last night and I believe that you are speaking about semantics. The 4 stroke cycle is the Otto cycle, not the ideal Otto cycle but still the Otto cycle.

My reference book is this;

The internal combustion engine in theory and practice by Taylor

http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=6992

And the High speed internal combustion engine by Ricardo

The latter has some interesting stuff on how tuning factors affect the shape of the curves.
RCVD
RCVDlink
Monday, March 22, 2010 9:12 AM
Ah i see, so theres an acutal and a ideal for the otto cycle. Thanks for clearing that up.

Yeh ive heard that book by mr. ricardo is pretty baller. I'll have to check that out one of these days
Mike Kojima
Mike Kojimalink
Monday, March 22, 2010 9:14 AM
I just happen to have an original copy of it. It took me 20 years of searching.
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