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Old 19th September 2009, 16:46   #1
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Default Car with four cylinder engine or three cylinder engine?

During the second half of eighties and nineties, three cylinder engines ruled Indian roads. But by the beginning of new millennium cars with four cylinder engines captured back the Indian market. But some recent introductions gives a feeling that whether the industry is swinging back to three cylinder engines - especially with the introduction of cars like Maruti Astar, Skoda Fabia, Maruti Estillo (2009) etc.

An average prospect is already confused with multiple market offers by automobile companies and now one more confusing choice - Car with four cylinder engine or three cylinder engine?

Apart form this basic question there are a dozen of queries which are unanswered or guided in the wrong directions. Some of them are:
  • Are we growing or going back to underpowered engines?
  • How these three cylinder engines are better than four cylinder ones?
  • Is it introduced to reduce manufacturing expenses?
  • Why some brands are charging more for cars with three cylinder on than a four cylinder ones?
Through this article I am trying to find out answers to many of these questions and try to explain the relative advantages and disadvantages of both.

Disclaimer: This article is not written to support any specific brands or offerings in the market. Moreover I am not an expert in automobile engineering; just sharing what I know. Feel free to contribute more in this subject.

No, we are not going back to underpowered engines; Indian car market is growing to maturity with better market offerings.

The power output of an engine is irrespective of the number of cylinders. Theoretically even a single cylinder engine can deliver the same power as a four or six cylinder engine, if designed accordingly. More than power delivered, the way in which the power delivered makes the difference and have greater impact in a practical scenario.

On manufacturing cost aspect I don’t think, the manufacturer is gaining nothing (other than some material cost) for delivering a three cylinder engine instead of a four cylinder one. Then why should the companies reduce the number of cylinders?

The answer is very simple – fuel efficiency.

Next to product cost, one of the most important aspect in Indian market is the mileage / fuel efficiency. People, especially the middle class want to reduce operating expenses by buying a car which gives better mileage. Lots of research happens to improve the fuel efficiency – one way to achieve this is by reducing the number of cylinders. But how? The answer is given in the “Fuel Efficiency” section.

Fantastic! Whether mileage is the only factor a customer is looking for?
No, mileage is one important factor, but there are many other factors like comfort, ride quality, acceleration etc, which all depends on the engine.

Lets evaluate two important performance aspects for both three and four cylinder engines:
a. Pulling Power
b. Fuel Efficiency

Pulling Power:

Generally people have an impression that a three cylinder engines are underpowered than four cylinder engines, especially when using AC. The feeling comes from experience in using certain vehicles with 800cc three port engines. 800cc engines are underpowered while using an Air-condition, not only because of the number of cylinders, but because of the displacement capacity. But a bigger three port engine is capable of delivering the same power as a four cylinder one. But the real difference is this:
  • A four cylinder deliver the power during all the four strokes in an engine cycle
  • A three cylinder engine deliver the same power in a different way.
Three Cylinder Engine:
In three cylinder four stroke engines, one engine cycle consists of two rotations i.e. while power is delivered from all the cylinders, the crank shaft rotate two times (720 degrees). The timing between the combustion phases of all the three cylinders is 240 degrees. But the combustion phase lasts only for a rotation of 180 degrees only. So for the time period of rotating remaining 60 degrees, power is not delivered by any of the cylinders and during that period the engine is driven purely by the on inertia of motion.

240 degrees = 180 degree combustion + 60 degree idle

So in every engine cycle, the power output is zero for 180 degree of turning

Note: For ease of representation and understanding, a 720 degree cycle is shown in a single cycle in the illustration. Please note the actual shape of the curve may vary from what is given I the diagrams.

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Observe the power output curve; even though the total power is same, the delivery is in an uneven way, much different from the ideal. So at lower rpm, there will be more engine idle time. Thus the engine is underpowered, which affects the pulling power. But as the rpm increase, the engine will execute more cycles and the idle time is reduced power will be much better. This makes the engine smoother to drive at higher rpm. Hence the car will be much better to drive in high ways than in a cities.



Four Cylinder Engine:
For four cylinder - four stroke engines, the power output characteristics are much better than a three cylinder one. The main difference is that in four cylinder engines; one combustion phase happens at every 180 degree, without leaving any ideal period. So the power output is continues. Hence the engine will have better response even at a lower rpm than a three cylinder engine. Thus the engine gives better power at a lower rpm and the car is easy to ride at a lower speed.

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Fuel efficiency:
It’s already mentioned that three cylinder engine gives better fuel efficiency than four cylinder engine, for the same power output and operating conditions. This section gives a theoretical explanation of how this happens by reducing a cylinder. To explain this two engines with same capacity (say 1L) is taken - one a four cylinder engine and the second one a three cylinder engine.

Note: I tried to illustrate this as simple as possible, those who don’t understand the calculations, please skip this section.

Assumptions
  • Both engines deliver same power
  • Same fuel
  • Operated under identical operating conditions
  • Stroke length / bore height of all cylinders are identical
  • No units are used as the case is hypothetical, so I put the measure as “unit”.
Formulas used:
Since the piston and cylinder cross section are circular.
Circumference of a Circle = 2 x pi x r
Area of a circle= pi x r x r

Case 1 : Four Cylinder Engine
The power output of each piston – cylinder unit is directly proportional to the displacement volume, i.e. the height and cross sectional area of the cylinder.

Cross Section Area
Radius of the piston = 1 unit (for simplicity)
Cross Section area of a single piston = 3.14 x 12 = 3.14 unit 2
No of cylinders = 4
Cross Section area of four pistons = 4 x 3.14 = 12.56unit 2


The contact area between the piston and the cylinder can be calculated by calculating the circumference of the piston

Circumference
Circumference of one piston = 2 x 3.14 x 1 = 6.28 units
No of cylinders = 4
Total circumference for all the four cylinders = 6.28 x 4 = 25.12 units


Case 2 : Three cylinder Engine

In order to generate the same power output in a three cylinder engine, the volume dispersed during combustion phase should be equal to the volume dispersed by all the four cylinders. Since we keep bore height constant in both engines, the only variable is piston radius. The new piston radius to generate the same power output is calculated as follows.

Calculation of new piston radius
Total cross section area of four cylinder engine = Total cross section area of three cylinder engine
4 x pi x r x r = 3 x pi x r x r
12.56 = 3 x pi x r x r
r = pi(12.56/ 3 x 3.14)
= 1.154 unit
Thus the radius of each piston should be 1.154 unit.

Total circumference of all three pistons = 3 x 2 x pi x r
= 3 x 2 x 3.14 x 1.154 = 21.74units

Ratio of piston circumference (surface contact) in a four cylinder engine compared to a three cylinder engine = 25.12 / 21.74 = 1.16

This means the surface area of the piston touching the cylinder in a four cylinder engine will be 1.16 times more than that in a three cylinder engine. Thus a four cylinder engine needs to work against more friction than a three cylinder engine in every engine cycle. Apart from friction inside cylinder, there are multiple frictions at every surface interfaces from crankshaft to valves. Thus a certain amount of output power generated by is used to compensate for these loses. As the number of cylinders goes up, more and more energy is utilized to overcome the frictional losses.

Thereby theoretically, a three cylinder engine is using less power to maintain the engine operation. Thus the engine need to generate only lesser energy during an engine cycle, for the same power output. Less energy generated means less fuel consumed. Thereby the fuel consumption in an engine cycle is less and the vehicle could give better mileage.

Conclusion
Other than this theoretical view, in a real life scenario, the performance, comfort and fuel efficiency of a car depends on many other factors starting from the aerodynamics to the passenger weight. There is no generalization that all three cylinder ones are fuel efficient and all four cylinder ones are better to drive. It depends on many other factors like the manufacturer, engine refinement, quality components, performance of the subsystems etc.

Finally, it’s a personal choice to go with a car with three cylinder or four cylinder. If somebody values more fuel economy, better operating expense and generally drives in fast lanes its better to have a three cylinder engine. At the same time if somebody is buying the car mainly for city driving, driving comfort and use AC most of the time, it’s better to have a four cylinder one. You decide…….
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Old 19th September 2009, 17:12   #2
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we also have a 2 cylinder choice in the form of TATA nano!
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Old 19th September 2009, 17:34   #3
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Excellent thread! I'd always prefer a 4 cylinder over 3 in my hatchback, simply because the refinement levels are far superior. The A-Stars 3 cylinder engine, advanced as it may be, has a rough idle, distinctly coarse edge when running and basically lacks the balance one would find...say...in a Ritz 1.2 4 cylinder engine. Both are from the same family of engines, but what a difference an additional cylinder can make.

A 3 cylinder is only apt when budget is tight. Cheaper to make, better FE.
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Old 19th September 2009, 18:13   #4
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One other factor i feel forgotten here is the crankshaft weights. In general any engine that produces more power strokes per rotation of the crank will mean lighter crank weights. Though this does not seem to be a big issue, a lighter crank weight means that it will store less energy on change of speed which means that more power will get transformed to the road quicker.

In normal human terms it means that any vehicle with lighter moving parts will accelerate faster. Thats why a free breathing Diesel cant still beat a restrictive breathing petrol.

But that said poeple are now moving away from too many cylinders. The V8s of the 90's in US are getting replaced with V6's and now Ford with Eco Boost seeks to further drop it to Turbo charged 4 Cylinders. Turbo charging has allowed every stroke of the engine to produce more power compensating for the extra weight that the cranks have to carry making them similar to a bigger engine in many ways. One more example is the yet to come 900cc engine from Fiat.

As we start accepting Hybrids as normal the acceleration/deceleration will be handled by the electric motor further reducing the need for engines to be accelerative. One example is the Honda Insight, only 3 cylinders to push the car but it can accelerate as fast as a normal Accord in US. There are still no turbo charged Hybrids which might mean we could see singles. With plugin-Hybrids the engine is removed off the entire equation allowing the designers to choose the most efficient engine design. A few busses in US already uses Gas turbines(micro turbine).
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Old 19th September 2009, 21:18   #5
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Red face Rough Engine

Hi I appreciate the technical inputs given by you but from a practically user point of view the 4 cyl engines are better...
1.Maruti 800 AC Tg 1996 to 2004
2.Alto lxi 2004 to date
Having driven and owned both these cars for a long period of time and comparing to wagon r (4cyl) I realised that the 3 cyl engines have a rough idle and rough when reved hard.As compared to wagon r having a very smooth idle and not all that rough when revved hard.They are naturally unstable 3cyl as compared to 4 cyl engines.
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Old 19th September 2009, 22:23   #6
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Default valves per cylinder

What is the impact of no. of valves per cylinder (we see mostly 3 or 4) on refinement/FE/power/torque?
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Old 19th September 2009, 23:01   #7
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Quote:
Originally Posted by anky View Post
Hi I appreciate the technical inputs given by you but from a practically user point of view the 4 cyl engines are better...
1.Maruti 800 AC Tg 1996 to 2004
2.Alto lxi 2004 to date
Having driven and owned both these cars for a long period of time and comparing to wagon r (4cyl) I realised that the 3 cyl engines have a rough idle and rough when reved hard.As compared to wagon r having a very smooth idle and not all that rough when revved hard.They are naturally unstable 3cyl as compared to 4 cyl engines.
"Unstable 3 cyl"? It depends on what you mean by "unstable". I have an Alto LXI and I cherish it for its FE. In fact, I also found it to be quite "stable" while on a recent inter-state trip. On the other hand, the tall-bodied Wagon-R is, in my opinion, unstable at high speeds and at sharp turns.

But yes, a 4-cylinder engine is considered more smoother than its 3-cylinder counterpart. Having said that, I love the sound of my Alto and for some strange reason, I have always preferred to 'hear' the sound of the engine that I drive and I don't like a silent engine. That may explain why I have always like the grunting sounds of jeeps in the past.

Regards,
Melvyn
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Old 20th September 2009, 00:30   #8
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3 cyl engines maynot be ideal for performance, but for regular driving, i dont see a prob.

The A-star and Estillo drive pretty well.

And not all 4 cyl engines are smooth, our Wagon R feels/sounds like crap when i revv it.

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Old 20th September 2009, 12:56   #9
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Quote:
Originally Posted by mohan41 View Post
What is the impact of no. of valves per cylinder (we see mostly 3 or 4) on refinement/FE/power/torque?
The more the number of valves the more air the engine ca gulp. It basically enhances the peak power and reduces lower end torque. If all engines were tuned the same then a 2 valve engine will be more fuel efficient in City conditions whereas the 4 valve engine will produce a lot more power. But in real world most 2 valve engines have high(er) lift where as the 4 valves have low(er) lift enhancing swirl.

About balance. A 3/6 cylinder engine is the most mechanically balanced engines in the lot. In a 3 cylinder engine, what ever the position the crank is in the three pistons balances each others weight(and torque) on either sides of the crank. In comparison in a four cylinder engine the engine is perfectly weight balanced only in certain positions(0,90,180 and 270 degs). But since 3 cylinders do not produce continues power it looses on smooth rotation of the crank. Thats why a V6 is considered superior as the crank is still a 3 cylinder equivalent but the power is much more continuous.

Saying that on the roads i feel that the four cylinders last much longer may be due to the fact that the fourth cylinder adds more power and better low end torque. Things get a lot more complicated when you consider the intake and fuel systems for a 3 cylinder engine Vs a four cylinder engine.

Again about WagonR Vs Alto. YOu have to remember that Alto has only a 796cc engine and WagonR has a 1100cc engine. Its not decisive that Alto is more efficient because its a 3 cylinder.

Last edited by arunmur : 20th September 2009 at 13:01.
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Old 20th September 2009, 14:42   #10
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Thanks GTO and all others who had given their comments.

Agree with Arunmur on the explanation on 4 valve cylinders. I will give a more detailed explanation on the reason behind four valves, in a later post.
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Old 21st October 2009, 10:22   #11
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Following are the cars with two and three cylinder engines in India.

Two Cylinder:
Tata Nano

Three Cylinder:
Maruti 800
Maruti Alto
Maruti Zen (old)
Maruti Zen Estilo (2009)
Maruti Astar
Skoda Fabia
Daewoo Matiz

Upcoming 800 cc engine cars:
Volkswagen Polo
Hyundai i10
Chevrolet spark

Four Cylinder:
All other cars have four cylinder engines or more.

Feel free to add, if I missed anything in this list.

Last edited by sunil8089 : 21st October 2009 at 10:23.
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Old 21st October 2009, 11:53   #12
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Quote:
Originally Posted by arunmur View Post
One other factor i feel forgotten here is the crankshaft weights. In general any engine that produces more power strokes per rotation of the crank will mean lighter crank weights. Though this does not seem to be a big issue, a lighter crank weight means that it will store less energy on change of speed which means that more power will get transformed to the road quicker.
I believe you meant "Flywheel" with "Crank Weights" together to store the energy for the subsequent cycles of the engine.

The power thrust on the crank shaft coupled with the pivoting of the connecting rod on the piston pin create a rotational force that is transfered to the flywheel bolted to the crankshaft.

Now the flywheel has it's own mass as does the crankshaft. It's that mass that helps speed remain stabilized( I mean constant speed). So the mass of the flywheel and crankshaft (collectively known as the rotating mass) keep engine revolutions at a constant.

Quote:
Originally Posted by arunmur View Post
In normal human terms it means that any vehicle with lighter moving parts will accelerate faster.
Its true but not in all the cases. A 3-cyl engine might have lesser rotating/moving parts which contributes to better FE but on the contrary a 4 cyl engine might have more rotating/moving parts but its an efficient machine in terms of better output and smoother, stable and better drive quality..

What point i want to put across is, just the number of rotating parts cannot be measure of performance...An efficient engine is the one which strikes the balance between torque, horse power, fuel efficiency and drive quality...
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Old 21st October 2009, 12:13   #13
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Now you know the reason why 6-cylinder cars are so much smoother. V8 V12 and W24 are of course in class of their own.

Even a five will be smoother than a four.
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Old 21st October 2009, 13:27   #14
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why would a 5 be better than a four.?
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Old 21st October 2009, 13:40   #15
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Excellent article! In general 4 cylinder engine's in comparison to 3 will be much more balanced and hence a smoother engine and power delivery. But i think technology is progressing big time that 3 cylinders can be made super smooth and 4 cylinders can be made super efficient.

Cost still i feel is a big factor, imagine the savings made by removing one whole cylinder and the associated parts. Raw material itself will make a huge difference if we consider the raw material cost these days, wont it?
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