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Old 22nd January 2010, 18:08   #16
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A lot of it has to do with tuning and basically how much power the manufacturer wants from the engine against how much mileage they want to extract from it. With mass market cars, they have efficiency pretty much at the top of their priority list. When technology advances, the equation changes. Suddenly it is possible to get more power AND more efficiency from a similar displacement.
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Old 22nd January 2010, 18:59   #17
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There is something called "Compression ratio"

This is one of the deciding factors in power and torque figures, irrespective of the engine size.

Compression ratio can be defined as the ratio of volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke

to

the volume of the combustion chamber when the piston is at the top of its stroke


A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency

If you see the sports bikes / cars, this compression ratio would be very high (greater than 10.5:1) unlike normal versions (lesser than 10:1)of same cubic capcity.
However, the higher octane fuel is needed - else it would result in engine knocking
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Old 22nd January 2010, 20:14   #18
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More than compression ratio, its compression pressure that actually creates torque/power.

The S2000 produces 120bhp/ltr and 109nm/ltr has a compression ratio of 11:1. The Veyron produces 123bhp/ltr and 156nm/ltr, but its compression ratio is just 9:1.

By using forced induction, you can make a low compression ratio engine produce much higher compression pressure than a NA build running super high compression ratio.

Which is why when an engine loses compression pressure due to a leak, its torque and power drops inspite of it retaining its original compression ratio.

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Old 22nd January 2010, 20:27   #19
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Formula 1 car even with smaller engine 2800 cc( correct me if I am wrong) produces more power than many bigger engines.
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Old 22nd January 2010, 22:00   #20
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Quote:
Originally Posted by toxicexpulsion View Post
Hi guys,

We the recent technological progress in automobile engines I begin to wander on the thought of this question.

One of the factors that influences a new car buyer is the size of its engine. However, there are many new engines on the block that are much smaller in terms of displacement but pump out much higher horsepower and torque.

Based on this premise, I question myself if one should really be bothered about displacement when comparing automobile engines.

Any inputs will be appreciated.
The number of cylinders that an engine contains is an important factor in the overall performance of the engine. Each cylinder contains a piston that pumps inside of it and those pistons connect to and turn the crankshaft. The more pistons there are pumping, the more combustive events are taking place during any given moment. That means that more power can be generated in less time.

Car manufacturers are constantly playing with all of the following variables to make an engine more powerful and/or more fuel efficient


Increase Engine Size -

More bigger engine means more power because you can burn more gas during each revolution of the engine. You can increase displacement by making the cylinders bigger or by adding more cylinders. Twelve cylinders seems to be the practical limit with a few exceptions..


Increase the compression ratio -

Higher compression ratios produce more power, up to a point. The more you compress the air/fuel mixture, however, the more likely it is to spontaneously burst into flame (before the spark plug ignites it). Higher-octane gasolines prevent this sort of early combustion. That is why high-performance cars generally need high-octane gasoline -- their engines are using higher compression ratios to get more power.


Stuff more into each cylinder -

If you can cram more air (and therefore fuel) into a cylinder of a given size, you can get more power from the cylinder (in the same way that you would by increasing the size of the cylinder). Turbochargers and superchargers pressurize the incoming air to effectively cram more air into a cylinder.


Cool the incoming air -

Compressing air raises its temperature. However, you would like to have the coolest air possible in the cylinder because the hotter the air is, the less it will expand when combustion takes place. Therefore, many turbocharged and supercharged cars have an intercooler. An intercooler is a special radiator through which the compressed air passes to cool it off before it enters the cylinder.



Let air come in more easily -

As a piston moves down in the intake stroke, air resistance can rob power from the engine. Air resistance can be lessened dramatically by putting two intake valves in each cylinder. Some newer cars are also using polished intake manifolds to eliminate air resistance there. Bigger air filters can also improve air flow.


Let exhaust exit more easily -

If air resistance makes it hard for exhaust to exit a cylinder, it robs the engine of power.

Air resistance can be lessened by adding a second exhaust valve to each cylinder (a car with two intake and two exhaust valves has four valves per cylinder, which improves performance -- when you hear a car ad tell you the car has four cylinders and 16 valves, what the ad is saying is that the engine has four valves per cylinder). If the exhaust pipe is too small or the muffler has a lot of air resistance, this can cause back-pressure, which has the same effect. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate back-pressure in the exhaust system. When you hear that a car has "dual exhaust," the goal is to improve the flow of exhaust by having two exhaust pipes instead of one.


Make everything lighter -

Lightweight parts help the engine perform better. Each time a piston changes direction, it uses up energy to stop the travel in one direction and start it in another. The lighter the piston, the less energy it takes.


Inject the fuel -

Fuel injection allows very precise metering of fuel to each cylinder. This improves performance and fuel economy.


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Old 23rd January 2010, 02:08   #21
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Thank you very much for your valuable inputs.

Lets look at the following example:

The Mercedes E class now comes with a new engine variant which is the 250CDI.

This particular engine comes embarrassingly close to its bigger brother (350CDI) in terms of performance (BHP and Torque) with only 4 cylinders and 2.15L of cubic capacity. The 350 CDI is mated to a 7G-Troinc transmission and the 250 CDI has been given the military medium 5 speed auto, agreed that the 350CDI is faster car but the 250CDI engine does a remarkable job. Its nearly as fast and torquey as the 350CDI in spite of its much smaller size.

Look at the Superb by Skoda. They have powered it with a 1.8TSI which IMHO sounds way too small for it. Similarly many other european car manufactures have followed the same path.

I cant stop but draw a parallel with computers. Remember the times with chip makers rolling out new processors with higher megahertz to speed things (Pentium 4) but today that has been abandoned and now we have processors with much slower megahertz (Core2Duo) and are much quicker with their predecessors. Whats evident is that they have dual cores and bigger cache memory which also consume lesser energy.

Likewise, we have smaller engines with twin turbo chargers and advanced fuel injection. These engines are not only more powerful but also very efficient.

I have a feeling that we are going to see this trend follow on in the near future given the changes in dynamics of the auto industry.

Looking for more answers.
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Old 23rd January 2010, 19:44   #22
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Plain example,


I drove bro-in-law's Ritz Zxi (1.2 K-series) for a 250 km round trip on NH-1, seemed silken smooth in all gears uptil 120 kmph but no compaison with Swift Vxi (1.3) beyond 120 kmph.

An apple to apple comparison of Swift 1.2 ( when available for TD) Vs Swift 1.3 can shed some more light.
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Old 24th January 2010, 14:57   #23
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Toxicexpulsion, a question indeed. And an extremely intriguing one at that. I'm going to rate this thread 5 stars before typing out my reply.

I will put it this way : With recent technological advancements, engine size is today less important than it was 20 years ago. Three of the best examples : The Skoda Superb. A 1.8 L engine only. Yet, thanks to direct injection tech, the low end torque is a match / better than the Accords 2.4 (an engine 30% larger). It's outright performance is also befitting of a 20 lakh rupee car, though the engine size itself is par for the 10 lakh segment (Altis, Civic etc. all with 1.8 liters).

Second example : The Fiat MJD with a variable geometry turbo. It's only a 1.3 liter engine, yet makes about the same power and torque as the Octavia 1.9 TDi (203 NM versus 210 NM). Now, the Octavia was as modern as diesels came in the year 2000. Yet, the Manza / Linea today with a 1.3 liter engine make nearly the same power as the Octavia did with a 1.9 L!

Third : The Merc E250. This is a 2.2 liter 4 cylinder engine that makes 500 NM of torque! It makes the 3.0 L V6 E350 CDI seem pointless with only a 40 NM additional. Plus, the E250, being a 4 cylinder, will be way more fuel efficient (and cheaper to buy). I've always insisted that the E250 will be Merc's bread and butter model in India. Can't wait to test it.

Of course, you also have VW's 1.4 TSI which makes 177 BHP and 250 NM of torque! For Christ's sake, it's a 1.4 liter engine at the end of the day.

Buttttttttttttttttt, I have to hand it to Tj123 for summing it up perfectly:

Quote:
Originally Posted by tj123 View Post
Theoretically speaking if all parameters are same a bigger engine with more displacement will produce more torque. Hence the engine will have better lugging ability.
If the bigger engine has the exact same tech as the smaller one, it's the bigger that will be superior / powerful in every way (except, weight and cost).

I reiterate, while bigger still means more power (all things remaining the same), the engine size is of lesser relevance today.

P.S. : Welcome aboard. I see this is your first thread. Look forward to many more interesting ones from you

Last edited by GTO : 24th January 2010 at 14:59.
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Old 24th January 2010, 15:22   #24
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Nice topic.

Following arguments to ponder:

This is based on the understanding that the only way a smaller engine stands up to a bigger engine is with Forced Induction.

Comparing bigger unstressed vs smaller stressed (FI) engine:

1) Wouldnt a bigger engine have a more linear toque/power curve?
Most smaller engines would need FI to achive similar actual displacements of a bigger engine and even the best Variable geometery turbo is not perfectly linear and still has some evidence of turbo lag.

2) Wouldnt a bigger engine last longer due to lower mechanical stress? Lower compression ratio would result in lower combustion temperature and pressure.

3) Considering the bigger engine is running a lower compression ration, it would require a lower octance petrol which would be easier to source. It will also handle adulterated petrol more easily.

4) The smaller engine should give you a better fuel efficiency as the actual displacement of the engine changes with rpm. Thus when puttering in the city a smaller engine will actually be running a much smaller actual displacement.

My understanding:
Small FI engine:

Pro:
Fuel efficiency
Lighter

Cons:
Maintaience
Susceptible to fuel adulteration
Power curve non linear

Big NA engine

Pro:
Easier to maintain
Power curve more linear
more reliable

Cons:
fuel guzzler
heavier

This is considering both engines built with today tech. Dont compare an old NA to a modern FI.
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Old 24th January 2010, 15:35   #25
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When it comes to normally aspirated engines engine size does matter.

Some normally aspirated production engines have got a torque per litre output as low as 60lb ft, which is nothing to bragg about in terms of performance. But there is reasons for it, which include climatic conditions, insurers demands, fuel qualities on a global scale where the engines have to run in any market without modification, the variety of changes of the chemical properties when mixing fuel from different manufacturers (because of different additive packages afecting each other), emission regulations (exhaust emissions as well as sound pressure level), production cost and last but not least the driver and the kind of driving.

A good 30% more torque per litre can be achieved with reasonable efforts.

Getting to 90lb ft normally aspirated requires a lot of effort and relevant fuel qualities.

So far the only way to get to 100lb ft or beyond is using forced induction.

On the normally aspirated engines when designed properly the torque output can be controlled to a certain extent. There is only point in achieving a comparatively high torque output when it is stretched over as much of the rpm range as possible. Peak bhp is irrelevant to acceleration.

The industries looks now more into small capacity forced induction engines because the advancements on normally aspirated engines come to a halt.

The advantage of small capacity forced induction engines is not only size but also weight. The smaller size allows also for smaller engine comaprtment, which brings down the over all vehicle weight other than the lighter engine.

What makes the performance of a car is not necessarily what is the performance of the engine.

Acceleration is determinde by the torque per ton on the wheels. And the further the torque band stretches the better it is.

One other disadvantage large capacity engines have is weight, which doesn't help the handling.
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Old 24th January 2010, 17:25   #26
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Let's start with fundamental law of thermodynamics:

Energy input(EI) - energy loss(EL) = energy output (EO)

EI is proportional to fuel intake (potential energy in gasoline/diesel + air). More the fuel intake (cylinder capacity) more the EI.

For same EI, EO depends on EL. Less EL more EO and vice-versa.

For bigger EI, EL is also more (bigger pistons, crank shafts etc- bigger frictional loss, inertial loss, heat loss- bigger engine surface area etc). Conversely, for smaller EI, EL is also smaller.

Besides above things, engines of yore, had comparatively more EL (due to manufacturing process, metallurgy, ignition technology etc) than the moderns engines. Hence, even a smaller engine of today, by managing EL (with better manufacturing process, metallurgy, ignition tech etc), is able to produce a better EO.

However, EI is still significant, since, even with current manufacturing process and technologies, we are unable to reduce EL considerably.

-BJ
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Old 24th January 2010, 17:34   #27
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While today's tech makes displacement seem redundent, I think that there is no replacement for displacement, especially between AMG, M division and RS quattro GMBH motors, all 5.0L plus with very high hp.

Though I am still a big fan of Corvette ZO6 7.0L LS7 V8 and ZR1 6.2 L supercharged V8. American muscle with brutal performance. And these motors only have 2 valves per cylinder. No, I am not kidding. They are pushrod operated OHV motors. No, I am again not kidding. But 0-100 in 3.7 and 4.2 seconds respectively does matter, doesn't it?

A little known but interesting fact is that the 7.0L LS 7 in the ZO6 can return 16mpg city and 26 mpg highway (according to US driving cycle) and despite producing 505 bhp, it evades the 2,500 $ federal gas guzzler tax.

And yes, right, I am a BIG fan of corvette.
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Old 24th January 2010, 17:42   #28
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Quote:
Originally Posted by GTO View Post
Toxicexpulsion, a question indeed. And an extremely intriguing one at that. I'm going to rate this thread 5 stars before typing out my reply.

I will put it this way : With recent technological advancements, engine size is today less important than it was 20 years ago. Three of the best examples : The Skoda Superb. A 1.8 L engine only. Yet, thanks to direct injection tech, the low end torque is a match / better than the Accords 2.4 (an engine 30% larger). It's outright performance is also befitting of a 20 lakh rupee car, though the engine size itself is par for the 10 lakh segment (Altis, Civic etc. all with 1.8 liters).

Second example : The Fiat MJD with a variable geometry turbo. It's only a 1.3 liter engine, yet makes about the same power and torque as the Octavia 1.9 TDi (203 NM versus 210 NM). Now, the Octavia was as modern as diesels came in the year 2000. Yet, the Manza / Linea today with a 1.3 liter engine make nearly the same power as the Octavia did with a 1.9 L!

Third : The Merc E250. This is a 2.2 liter 4 cylinder engine that makes 500 NM of torque! It makes the 3.0 L V6 E350 CDI seem pointless with only a 40 NM additional. Plus, the E250, being a 4 cylinder, will be way more fuel efficient (and cheaper to buy). I've always insisted that the E250 will be Merc's bread and butter model in India. Can't wait to test it.

Of course, you also have VW's 1.4 TSI which makes 177 BHP and 250 NM of torque! For Christ's sake, it's a 1.4 liter engine at the end of the day.

Guys correct me if i'm wrong.

Two cars with same displacement(cc), but one car is faster than other. That means some advanced technology used, which means engine is performance tuned. So in a long term the car with more power will last less than car without mod.
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Old 24th January 2010, 19:26   #29
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Quote:
Guys correct me if i'm wrong.

Two cars with same displacement(cc), but one car is faster than other. That means some advanced technology used, which means engine is performance tuned. So in a long term the car with more power will last less than car without mod.
Its not that simple. Displacement is just the volume that air/fuel can occupy inside the engine. But even though all 2ltr engines have the same volume, what needs to be seen is the density of the air/fuel being packed into this space.

When you mod a stock engine with a ffe, headers and a performance filter, you are not changing the design or the displacement of the engine in anyway. But you still see a rise in power and torque. Thats bcos these basic mods help the engine breath better and suck/pack in more air/fuel into a specified space. This helps the engine push the pistons with greater force which leads to more torque and eventually power.

Each mod brings in a rise in torque/power. But there is a limit to how much air/fuel can be packed into an engine naturally. This is where "Forced Induction" takes over.

FI lets the engine not only produce more power but also loads of torque. The Skoda 1.8 RS has 210nm of torque but the Civic only manages 175nm, even though they have the same displacement.
Quote:
When it comes to normally aspirated engines engine size does matter.
Engine size matters irrespective of NA or FI.

If a 2ltr FI engine produces 100lbft/ltr (200lbft), a 4ltr engine putting out 100lfbt/ltr will give you 400lf/bt.

So from a technical point of view, bigger is always better.

But from a practical point of view, a smaller engine with high specific output makes more sense.

Shan2nu

Last edited by Rudra Sen : 25th January 2010 at 12:43. Reason: Post merged
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Old 25th January 2010, 11:39   #30
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Just gave this thread a 5 rating, as you and the rest have rightly pointed out that the gap between displacement based power output and small capacity based power output is closing fast. In todays world coming to a 100bhp/ltr output is now considered easily within reach because of of better VE than a NA motor.

A lot however comes to the fact that smaller cars have smaller engines and hence the bhp/ton is not compromised. However you will note that higher displacement engines are typically designed for high torque and low rpm leading to longer engine life vs. smaller engines with forced induction used to be prone to failure (note I said used to), todays technologies are allowing the newer engines to be built better rather than just a turbo / supercharger being added hence ensuring better engine life.
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