Few technical questions

[aarganesh]

Hello Experts
Plz help me understand the following...

a. What is compression ratio? Does it refer to the pressure generated by the piston's upward movement towards the spark plug or the pressure generated after the spark plug has ignited the fuel and air mixture?

b. Why is the compression stroke appear constant in most of the vehicles like 9.1:1 or 10:1 etc? Can this be changed?

c. Can a four stroke engine be rebored as a 2 stroke engine? I'm not sure if it is common why is that so?

d. I may sound cranky in this but still...can gears be attached to the crank shaft so that the number of revolution from the piston be increased? Which will inturn cause more torque or increase the overall revolution including the gearboxes?

PS - I may sound silly on few of my thoughts because I'm not formally educated automotive.

Thanks in Advance
Regards
Ganesh .R

[Ridenour]

welcome to the forum.

a) compression ratio is how much the volume of each cylinder is compressed. Example, if you had a 1 liter displacement per cylinder, and after compression it was down to 0.1 liters, you would have a 10:1 compression ratio.

B) I don't really understand what you're asking. In order to change your compression ratio, you would have to somehow alter your piston's stroke length, or change the displacement of your heads, ect.

C) I'm not sure you understand what a 4 stroke and 2 stroke engine is. Read this - it will allow you to undestand the differences between the 2 engines:
http://science.howstuffworks.com/two-stroke1.htm

D) I'm not sure what you're asking. Could you be more specific?

[beef_bourito]

Compression ratio is the volume from bottom dead center divided by the volume at top dead center (when the piston is at the very bottom and the very top)

Compression ratio can be changed by altering one of these two things, either increasing the stroke and keeping the same head volume or deminishing the head volume. higher compression will give you more power, torque and higher revs but if you increase it too much and don't use higher octane fuel, you may get detonation and bye bye engine.

I'm sure it's possible to modify a 4 stroke to be a 2 stroke but it would be extremely difficult. but why would you want to do it, you will get more power but much less fuel economy and a smaller availability of parts.

you don't need to do this, you can swap out your gears in your transmission. just for simplicity sake, think of you transmission as the back gears on a bike without the middle gears(the ones right at your feet) if you just swap out those gears for smaller ones you get a higher top speed, more fuel economy but decreased acceleration. on the other hand, if you make them bigger, you get more acceleration but less top speed. plus you can get a new rear end (for rear wheel drive cars) that has a different gear ratio to bump up acceleration.

hope i helped, if you're confused, just ask me to clarify.

[curtis73]

Originally Posted by aarganesh

a. What is compression ratio? Does it refer to the pressure generated by the piston's upward movement towards the spark plug or the pressure generated after the spark plug has ignited the fuel and air mixture?

The other boys took pretty good care of that one.

b. Why is the compression stroke appear constant in most of the vehicles like 9.1:1 or 10:1 etc? Can this be changed?

This one was pretty well covered as well, but I want to add to what beefbourito said; you can increase compression by increasing the displacement of the engine by any means, not just stroke. Using a previous example, if one cylinder displaces 1L at its largest and 0.1L at its smallest, you have 10:1 compression. If you left everything else alone but increased the bore so it displaced 1.1L, and it would still displace 0.1L at its smallest for a compression ratio of 11:1

c. Can a four stroke engine be rebored as a 2 stroke engine? I'm not sure if it is common why is that so?

in a word, no.

d. I may sound cranky in this but still...can gears be attached to the crank shaft so that the number of revolution from the piston be increased? Which will inturn cause more torque or increase the overall revolution including the gearboxes?

You could, but it would waste energy. If you overdrive the crank by 2:1, you would be cutting the torque in half. The overall output would remain the same. You would also just end up having to change the final drive ratio or transmission ratios to make it so you can use the new RPMs you've just made. Just overdriving the crank and changing nothing else would make your highway RPMs half of what they used to be and you'd have no power. Not to mention, it would be multiplying the crank's inertia by the square of its weight making all that energy go to waste. Plus, there would be extra friction, which also increases by the square of its RPM.

[aarganesh]

Hello Friends
Thanks for the clarification.

Can you explain little more in increasing the compression ratio? What may be the pros and cons of increasing/decreasing it?

I'm very sorry that I completely messed my question on reboring.

A two stroke engine can be rebored so that the cylinder head becomes slightly bigger insize due to which the piston size also needs to be changed. This reboring may be done after 100,000 Kms or so depending on the scratches on the cylinder head (commonly called as engine seize here in India). When doing this sometimes, we do something called porting (not sure what this is) but the outcome is the engine gets more power subsequently reducing the mileage.

Well here is what I intended to ask...Can a 4 stroke engine be also...
a. Rebored just like the above mentioned 2 stroke engine?
b. Ported like above?

Thank you very much
Ganesh .R

[curtis73]

Originally Posted by aarganesh

Can you explain little more in increasing the compression ratio? What may be the pros and cons of increasing/decreasing it?

Sure. The compression ratio changes how much squeeze you put on the fuel/air mixture before the spark happens. The higher the compresion ratio, the more you squeeze, and the hotter the charge becomes. This has its benefits and drawbacks. The main benefit is that the increased temperature of the air/fuel before the spark makes it burn more efficiently. The engine is able to convert more of the energy stored in the fuel into power with higher compression. This is not to be confused with fuel efficiency, but it can be related. The main drawback is that if you squeeze too much, the fuel can ignite on its own, or have a flame speed that is too fast. When the flame starts before its supposed to, the flame front hits the piston like a hammer causing ping. Its also called detonation, knock, dieseling, etc. The trick is to use the right compression ratio for your application. Typically engines made of aluminum can use a little higher compression ratio since the aluminum soaks heat away faster. For instance an engine with iron components using 9:1 compression might operate the same as the same engine cast in aluminum with 9.5:1. There are many factors in what compression ratio should be chosen, but street engines usually fall between 8:1 and 11:1. The compression ratio is also a determining factor in the octane rating of fuel your engine requires. Octane reduces the fuel's tendency to self-ignite when you put the squeeze on it during compression.

I'm very sorry that I completely messed my question on reboring.

Don't be sorry Your question was valid.

Well here is what I intended to ask...Can a 4 stroke engine be also...
a. Rebored just like the above mentioned 2 stroke engine?
b. Ported like above?

a. yes. Its pretty common when a 4-stroke engine has scratched cylinder walls or bad rings to rebore it to a bigger size. They can also be bored larger and then have sleeves like tubes pressed in to return it to original size. But (at least here in the states) the cost of bigger pistons is usually cheaper than having sleeves pressed in.

b. yes, you can port either one and the theory is the same for both styles of engine. The bigger the port, the more air/fuel can get in there, and therefore more power. The only difference is where the port is located; in the head for a 4 stroke and in the block for 2 stroke.

[aarganesh]

Thank you very much. That helps me understand the complications involved in compressions & doubts about porting and reboring 4 strokes.

I've a question on multipl cylinder engines. Plz let me know if I can post it here or I will start a new thread.

Here is the question...
Lets take an example of a 4 cylinder engine (be it inline or V shape). I lately came to know that the pistons will be at different stages and each connecting rod will rotate the crank shaft at different point in time. At no point of time all four will rotate the crank shaft (which I was thinking).

Based on the above here are my questions...

a. For this type of engine, I guess, the maximum speed of all the 4 connecting rod will be the same. So how does increasing the number of cylinders will increase the speed of vehicle or is this just my assumption that increasing the number of cylinders will increase the speed of vehicle (assuming all other factors remain the same like weight of car, # of passengers etc).

b. Why is that the engines are designed in such a way that all four connecting rods rotate the crank shaft at different point in time rather than all of them rotating at one time?

c. I think (or assume), if all 4 connecting rods rotate the crank shaft at one point in time the load on the engine is distributed to all 4.

Plz correct me if I'd posted something incorrectly or have not phrased my questions correctly. I will be happy to rephrase my question if anything is inclear.

TIA
Ganesh .R

[curtis73]

Originally Posted by aarganesh

I've a question on multipl cylinder engines. Plz let me know if I can post it here or I will start a new thread.

This is a great thread, let's keep it going

Lets take an example of a 4 cylinder engine (be it inline or V shape). I lately came to know that the pistons will be at different stages and each connecting rod will rotate the crank shaft at different point in time. At no point of time all four will rotate the crank shaft (which I was thinking).

That is correct.

a. For this type of engine, I guess, the maximum speed of all the 4 connecting rod will be the same. So how does increasing the number of cylinders will increase the speed of vehicle or is this just my assumption that increasing the number of cylinders will increase the speed of vehicle (assuming all other factors remain the same like weight of car, # of passengers etc).

Increasing the number of cylinders is a way of increasing engine displacement (among other things). Its just that there are certain points at which designing more or fewer cylinders makes sense. For instance in large commercial trucks which need a lot of engine displacement, a four cylinder doesn't make sense. The cylinders would have to be so big that the pistons would be very heavy, and the vibrations the engine would put out would not be pleasing. In a small car, a four cylinder makes sense as a way of maximizing displacement in a small area.

b. Why is that the engines are designed in such a way that all four connecting rods rotate the crank shaft at different point in time rather than all of them rotating at one time?

Two main reasons: 1) so that the force from one piston can move the other three through their other strokes. If they all fired at once, the engine would only operate at a speed fast enough that its inertia would carry it through all four strokes. However, this does occur in one-cylinder, 4 stroke engines. Since there is only one cylinder, it has to use inertia to carry it through. Because of this they have a narrower range of applications. 2) because of #1, mostly for the sake of smoothness. An engine with all of its rotating weight moving in the same direction would cause pretty violent shaking. Not only would it make you angry, it would shorten the life of the engine

c. I think (or assume), if all 4 connecting rods rotate the crank shaft at one point in time the load on the engine is distributed to all 4.

While your logic is correct, most of the load on the engine is carried in the crankshaft. So in this case, spreading out the loads from the pistons into four distinct events helps even out the load instead of all four hitting at once.

[beef_bourito]

I was going to say the same thing but add something for the adding cylenders. When you add more cylenders it makes the engine run smoother. I think this helps with resistances or something so that it allows the engine to rev higher. Formula one uses V10 engines (they're being cut down to 8 next season) that only displace 3 litres. the extra cylenders i think help it to rev higher (up to 15,000rpm, let's see that out of a civic)

[DRW1000]

Originally Posted by curtis73

This one was pretty well covered as well, but I want to add to what beefbourito said; you can increase compression by increasing the displacement of the engine by any means, not just stroke. Using a previous example, if one cylinder displaces 1L at its largest and 0.1L at its smallest, you have 10:1 compression. If you left everything else alone but increased the bore so it displaced 1.1L, and it would still displace 0.1L at its smallest for a compression ratio of 11:1

.

Curtis73,

I am confused...........If you had the 1L to 0.1L cylinder bored such that it displaced 1.1L would it not compress to 0.11L (or something close) ?? I realize the top of the cylinder is not flat topped but I seem to feel that The larger bore would cause a larger volume when compressed than the original bore.

Set me straight. (I know I am missing something...I just don't know what)

[curtis73]

You are right in practice. For the sake of simplicity in the demonstration I was assuming a flat top piston at zero deck and actually no clearance to the head. In practice there is more combustion space than just the chamber, but the math is tougher

[aarganesh]

Hello Experts
Thanks.

--------------------------------------------------
Increasing the number of cylinders is a way of increasing engine displacement (among other things). Its just that there are certain points at which designing more or fewer cylinders makes sense. For instance in large commercial trucks which need a lot of engine displacement, a four cylinder doesn't make sense. The cylinders would have to be so big that the pistons would be very heavy, and the vibrations the engine would put out would not be pleasing. In a small car, a four cylinder makes sense as a way of maximizing displacement in a small area.
--------------------------------------------------

I'm sorry that I'm unable to understand completely on the above. Could you please enlighten me more on the above?


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Two main reasons: 1) so that the force from one piston can move the other three through their other strokes. If they all fired at once, the engine would only operate at a speed fast enough that its inertia would carry it through all four strokes. However, this does occur in one-cylinder, 4 stroke engines. Since there is only one cylinder, it has to use inertia to carry it through. Because of this they have a narrower range of applications. 2) because of #1, mostly for the sake of smoothness. An engine with all of its rotating weight moving in the same direction would cause pretty violent shaking. Not only would it make you angry, it would shorten the life of the engine
--------------------------------------------------

So if I understand right, if all 4/6 connecting rod tries to work on the crank shaft at the same time, then, the inertia would try to break one of the component like crank shaft or connection rod or sometimes the piston itself. Am I right?

TIA
Ganesh .R

[sunfire_starter]

Originally Posted by aarganesh

Hello Experts
Thanks.

--------------------------------------------------
Increasing the number of cylinders is a way of increasing engine displacement (among other things). Its just that there are certain points at which designing more or fewer cylinders makes sense. For instance in large commercial trucks which need a lot of engine displacement, a four cylinder doesn't make sense. The cylinders would have to be so big that the pistons would be very heavy, and the vibrations the engine would put out would not be pleasing. In a small car, a four cylinder makes sense as a way of maximizing displacement in a small area.
--------------------------------------------------

I'm sorry that I'm unable to understand completely on the above. Could you please enlighten me more on the above?

Curtis is saying that the number of cylinders in an engine really depends on what veichle you are in. In a big commerical truck it would be pointless to have a I4 in one of those because of all the engine space avalible under the hood and the smallness of an I4 engine and plus there wouldn't be enough engine displacement (power) to move such a heavy load.So you would need more cylinders to get the power you need something like a V12(I don't know if that is what they use just a guess). With a regular car you could use a I4 because the engine displacement would move the car and it fits in a small car engine bay, and it is just a small load compared to a commerical truck. I hope I clarified it for you and just didn't confuse you more.

[aarganesh]

Ok Thanks. I think I'm getting it. It would take sometime for me to understand.
I know there is a relationship between the cylinder chamber (including piston, crankshaft, connecting rod) and the RPM. But I cannot visualize it. Could you please help me relating them.

TIA
Ganesh .R

[curtis73]

Originally Posted by aarganesh

I know there is a relationship between the cylinder chamber (including piston, crankshaft, connecting rod) and the RPM. But I cannot visualize it. Could you please help me relating them.

TIA
Ganesh .R

In general, larger displacement engines have lower RPMs for the main reason of inertia. If you have two indentical displacement engines, one with a large bore, short stroke and the other with a small bore, long stroke, the first engine should be more capable of spinning faster before it fails for the same reason; inertia. Although the pistons would be larger, they don't go as far or move as fast and the inertia would be less.

As far as their power production is concerned, that is more of a function of the other components that make up the engine.