Technical terms

[Acemundo]

anyone care to explain what are gearing and compression ratios? is it bigger ratios better? i can fathom a guess, but i thought it would be better to hear it from the experts' mouth.

 

also i was looking at the specs of m3 1.6 5mt and the 4eat.

 

gear ratios 1st 3.416 vs 2.816 , 2nd 1.842 vs 1.497, 3rd 1.29 vs 1.00 4th 0.972 vs 0.725, 5th 0.820 vs na and rev 3.214 vs 2.648. Since all gear ratios of 5mt is higher than 4eat, how come the overall final gear ratio reads 4.388 vs 4.416? is it because the final gear ratio is an average so that 5mt gear ratio gets drag down because it has to be divided by 5 (instead of 4 for the 4eat)? But if the final gear ratio is an average it also does not make sense since 4.388 is higher than any of the gear ratios of the 1st - 5th gear.

 

how does cams and cylinders and valves relate to each other? any website that explains these?

[BoneStock]

Hi Ace,

 

Gearbox has an input shaft that is connected to the clutch(for manual) or the torque convertor(for auto), and an output shaft that is connected to the final drive. Gear ratio is the ratio of the number of gears for input versus output....you can take this for manual tranny. For auto tranny, the principle is still very much the same but the construction and operation is very complicated. Usually a set of gears in the auto tranny is good for 3 different speeds. For manual, one set of gears is meant for one speed only. Quite difficult to put into words, these are best illustrated with diagrams.

 

Compression ratio is the how much the air is compressed from induction stroke to compression stroke. In other words, when the piston is at the bottom of the cylinder(or BDC), the volume of air in the cylinder will be the greatest...all this air will be compressed to form the small little space at the top of the cylinder when the piston is at the top of the cylinder(TDC). Again, all these are best illustrated with diagrams...my england beri bad....

 

You can find more info at http://auto.howstuffworks.com/

 

Final drive is another set of gears between the gearbox and the drive shaft.

Edited January 24, 2004 by Vtecc

[BoneStock]

pistons are driven by a huge shaft called cranshaft. Crankshaft is connected to the camshaft with timing belt or timing chain. The camshaft(or cam) controls the opening and closing of valves...

 

Cylinders are the round hollow spaces in the engine that the pistons reside.

[Acemundo]

so my guess about the gear ratio is correct. the bigger the ratio the more efficient?

 

but as for compression ratio, it is still a bit unclear to me since how much the air is compressed does not equate to how much work done or efficiency?

 

oh so there is another set of gears in the final drive.......so when comparing specs of car, is it better to choose higher gearing ratios in the respective gears or the final drive?

[Acemundo]

ok this diagram is very helpful in my understanding.

 

so am i right to say cylinders are for output? so if we decrease the number of cylinders what is the impact?

 

valve can be for both input and ouput? most cars has got 16 valves and dual cams and 4 cylinders. what does more no or less no of valves affect?

 

does 1 cam always work with 1 piston? if that's the case single cam engines are not powerful enough?

[Yttrium]

The higher the compression ratio, the denser the fuel/air mixture is. This increases the mixture temperature and the thermal efficiency of the combustion process.. resulting in higher power. But this is only up to a certain point. For petrol engines the problem with too high a compression ratio would be detonation.

[Acemundo]

that makes sense.......so does the spark plug have any impact on compression ratio?

 

or rather is it that you can only see the efficiency of the combustion by the richness of the mixture on the spark plugs but not that the spark plugs affects the richness of the combustion?

[Yttrium]

Spark plugs have no effect on the compression ratio. It affects the combustion process by how good a spark it can provide to ignite the fuel/air mixture.

 

As for "richness/leaness" this refer to the ratio of the fuel/air mixture... whether the air/fuel mixture is higher (lean) or lower (rich)than the stoichiometric ratio ( 14.7 parts air to 1 part fuel, "ideal" theoretical ratio).

Edited January 24, 2004 by Yttrium

[Acemundo]

actually i am curious how car makers can vary their compression ratio?

[Saaber]

The higher the compression ratio, the better the power but in return the petrol used have to be higher octane to prevent pinking....

[Soya]

higher gearing is not necessarily better esp in urban driving. imagine if a car's gear ratio is too high, the driver lands up using a lower gear more often to keep the revs within the powerband. on the other hand, if the ratios were too low, the car would be revving it's nuts off and probably run out of breath very soon on high speeds.

[Acemundo]

oh i see.....

 

so in their design of the engine, it will have to use higher octane fuel.......actually i wonder how engines are designed to interact with different octane levels of petrol?

[Acemundo]

ok but you are referring to the division of gears within a same car. but if we are comparing 2 cars with 5 gears, the gearing ratio is preferred to be higher right?

 

or is it good to select lower ratio for sg context?

[Soya]

it simply means the car wif the higher ratio has longer legs for less buzzy high speed cruising.

[Acemundo]

haha.......i get wat u mean.

 

just like if you are a coach and looking for sprinter, you will go for those with legs packed with muscles.

[Soya]

wah...cheem. but it doesn't mean more muscles. kinda like if u run a marathon, better to be the taller guy and have longer legs than a someone who is shorter. a bit less effort in the long run.

[Yttrium]

actually i am curious how car makers can vary their compression ratio?

 

 

 

 

The compression ratio is determined by the swept volume of the piston's movement vs the cylinder head volume. So, to increase compression ratio they can either make the cylinder head volume smaller... or have a bigger cylinder bore or stroke so that the swept volume is larger... Some ppl who modify engines will weld bit of metal in the cylinder head to make the volume smaller... thus increasing the comp ratio..

[Yttrium]

Depends on the engine's available torque oso lah... if engine is too puny and the gearing is too high... acceleration will be poor.. so it's a balance between economy and reasonable performance. Just like one too different guys riding bicycles.. you can give them very high gearing for high theoretical topspeed... but the skinny weaker guy may not be able to push the pedals effectively to even get it moving.. whereas a big strong muscular guy could. Make sense?