[Emvi]

I have always been curious to know why are engine displacement and power output figures not a "perfect" number in automobiles!? Elaborating a bit further, when I say "perfect" I mean, from a 100cc bike to a 7.0L engined truck, neither the bike's engine is perfectly 100cc nor the truck's engine displacement is exactly 7.0L and so is the case for all the vehicles with various engine capacities. In addition to this, the power output figures throw quite interesting numbers too.

It is understandable that on occasions manufacturers deliberately design and manufacture engines with "imperfect" displacement figures in order to comply with regulations that are in place in some countries/regions. For example, in India, the engine displacement for a "small" car shouldn't exceed 1.5L for a diesel and 1.2L for a petrol to comply with the "small" car tax regulations. So, invariably the engine displacement is kept below the limit atleast by a cubic centimetre (CC)! A similar exercise was followed by Mahindra to comply with government regulations when they downsized the XUV 5OO engine to accommodate it under the sub 2L engine classification.

I have googled a few times and have not found a convincing explanation yet for the "imperfect" engine displacements. I found one of the explanations interesting though.
πr²h is the mathematical formula to calculate the volume of a cylinder, where r=radius, h=height of the cylinder and the value of π=3.1428. Since π is not a whole number, the volume of a cylinder or in this case, the volume/displacement of an automobile engine is never a perfect number! Interesting, logical! So each and every cylindrical component in this world is imperfect dimensionally or in other words, the volume of a cylindrical component is never a perfect/whole number? I honestly don't know.

The way certain engines are classified by the manufacturers is quite bizarre as well. Curiosly enough the legendary 1.3L multijet Fiat engine is actually 1248 cc! So it is essentially 1.25L and for the purpose of classification, logically it should be 1.2L, No? Another such example is Kia/Hyundai 1.4L GDI engines, which are actually 1353 cc! There are plenty of such examples in our country and around the world.

Now coming to the power output figures in terms of BHP/PS and torque in terms of Nm, more times than not, these figures are not "perfect" numbers, are they? So, while designing and developing an engine, what is the thing that the engineers focus on? Do they develop an engine with displacement as the constraint and see how much power an engine can develop or do they decide on the power output requirement and then arrive at the displacement? Either way, it requires engineering and mathematics and I repeat why are these figures not perfect!? Consider Hero Splendor- 97.2 cc engine capacity, 5.9kw or 7.912 BHP of power and 8.05 Nm of torque! Wow!

It is important to note that the dimensional deviations within prescribed limits are always allowed in the manufacturing industries owing to the practical issues in the machining processes, which are generally called machining tolerances. This may also add up to the imperfections in the dimensions and power output figures, of course not significantly. This further pops up a question in my mind. Now again taking Hero Splendor for the purpose of discussion, if each and every Splendor manufactured is identical in terms of engine capacity an power output figures, 97.2cc and 7.912 Bhp!? If they are not, by how much?

Precision engineering is one of the key elements in an automotive industry. Ironically though, certain noteworthy aspects in an automobile are far from being precise/perfect, like the one in discussion, atleast numerically.

Your insights BHPians.

[SS-Traveller]

Strange question.

Engine designers are limited by many factors, not just the exact number of 'cc' that may be required. Very few engines today are designed from the ground up with all components developed specific to that engine. And nor do cars get designed around engines - it's the other way round, where engines need to fit cars.

So an "engine's engineer" would be limited by the space in the engine bay, the engine components he can procure at best prices, and a hundred other things, while remembering the brief that it all has to measure up to 1200 cc or whatever other legal restriction.

As to your wanting the precise displacement or power / torque output to be advertised rather than those figure to be rounded off, well, that is mentioned in the technical specifications - but how does that help most technically challenged car buyers?

[Jeroen]

Certainly when it comes to engine displacement it is completely useless to aim for a round number. The required displacement and power output are associated / dependening on a whole lot of other factors as already mentioned by SS Traveller.

If anything from a production point of view something like a bore is nice to have as whole number, which will always lead to an not rounded number of displacement obviously.

It is not a result of tolerances in the machining process. Dialing in 20mm in the milling machine is less prone to mistakes, then say 19,87625568mm

Production tolerances are so small they don’t even show up in these sort of calculations. Bore and stroke are machined within less than thousands of a mm.

In essence they are just rounded numbers, because anything else doesn’t matter.

πr²h is the mathematical formula to calculate the volume of a cylinder, where r=radius, h=height of the cylinder and the value of π=3.1428. Since π is not a whole number, the volume of a cylinder or in this case, the volume/displacement of an automobile engine is never a perfect number!

The discplacement/volume is not calculated using the height of the cilinder, but primarily the stroke of the piston. Which is always less. And even that is not entirely true. The displaced volume is also depending on the volume trapped between the cilinder / piston and top piston ring (assuming no leakage). Also, your piston might not be flat on the top. It can be rounded, or have indents. so you need to add for that as well. So you have to add or subtract a bit.

Here a few examples:







You can calculate it if you have all the dimension. Or you can measure it. Put the piston in the bottom end position. Take the spark or injector out, fill with oil to the brim, now carefully rotate the crank through 180o to get the piston to top bottom position. You need to carefully measure the oil that spills out; the volume of oil displaced is the exact volume/displacement of that particular piston/cilinder.

I have done it a few times at Naval Engineering school on some marine diesels. It was interesting to measure the differences between cylinder, as an indication of potential wear without having to take the cilinder heads off.

Jeroen

[Dr.AD]

Originally Posted by Emvi

I have always been curious to know why are engine displacement and power output figures not a "perfect" number in automobiles!? Elaborating a bit further, when I say "perfect" I mean, from a 100cc bike to a 7.0L engined truck, neither the bike's engine is perfectly 100cc nor the truck's engine displacement is exactly 7.0L and so is the case for all the vehicles with various engine capacities. In addition to this, the power output figures throw quite interesting numbers too.

First of all, "imperfect" is not the right word for what you are trying to say. You probably meant not a round number. There is no such thing as an imperfect number. "Imperfect" almost sounds like something went wrong or someone missed something or someone is trying to pull of a fraud. None of that is the case.

Originally Posted by Jeroen

Certainly when it comes to engine displacement it is completely useless to aim for a round number.

That is the absolutely correct answer and the true reason why engine capacities are not round numbers.

Further, this has nothing specific to engine capacities and other engine specifications. In any engineering design, the final specifications and performance metrics are output of a very complex (and nonlinear) relationship among so many design parameters. The design has to follow several important design constraints, and after having designed the parameters to meet all the constraints, the net output specification will not necessarily be a round number, And if one of has to try to get a round number, while it may be technically possible, there is zero advantage in that. Why mess up a beautifully optimized design set just to get a round number in the end?

This is true for almost anything. When an engine's power is rated at 140kW (as an example), it will not necessarily be an exact round number of 140.00. When a mobile phone's battery is rated at 3000mAh (again as an example), most likely that number is an approximation and a rounded up/down figure from the true number which will be slightly different from precise 3000.00. Even fuel tank capacities are an approximate numbers. Fuel tanks shapes are very complex, and if one has to design a tank with exactly 50.00 liters of capacity, it will require a lot of additional (and unnecessary) design work with zero advantage in return. How does it matter if the real capacity is actually 49.95l or 50.05l and then rounded to 50l?

To summarize, engineering design is done to satisfy many constraints. Every new constraint added increases the design efforts and costs (sometimes significantly), and worse, it may require trade-off on some other constraints. While technically it may be possible to design engines with round numbers, that will be an additional constraint that will increase design efforts and costs, without any compensating benefit in return.

[arzala]

Here is another perspective.

https://mechanics.stackexchange.com/...n-exact-number

[Emvi]

Originally Posted by SS-Traveller

As to your wanting the precise displacement or power / torque output to be advertised rather than those figure to be rounded off, well, that is mentioned in the technical specifications - but how does that help most technically challenged car buyers?

It is not that I want the the manufacturers to advertise the exact displacement/power figures, rather I am trying to understand why they are not close to the classification mark if not exactly that number. And the lesser informed vehicle buyers in the rural and semi urban usually are aware of the displacement of their vehicles more than anything. Displacement is a great marketing tool. This reminds me of an incident which happened more than a decade ago. My colleague back then, an electrical engineer, proudly claimed his Apache 160 to be FAR superior to my Yamaha Fz just because his bike engine was about 6CCs bigger than mine! (Apache was infact about 1bhp more powerful, which the gentleman had no idea about)

Originally Posted by Jeroen

The discplacement/volume is not calculated using the height of the cilinder, but primarily the stroke of the piston. Which is always less. And even that is not entirely true. The displaced volume is also depending on the volume trapped between the cilinder / piston and top piston ring (assuming no leakage). Also, your piston might not be flat on the top. It can be rounded, or have indents. so you need to add for that as well. So you have to add or subtract a bit.

You can calculate it if you have all the dimension. Or you can measure it. Put the piston in the bottom end position. Take the spark or injector out, fill with oil to the brim, now carefully rotate the crank through 180o to get the piston to top bottom position. You need to carefully measure the oil that spills out; the volume of oil displaced is the exact volume/displacement of that particular piston/cilinder

Superb explanation sir! Thank you
Yes, πr²h is a general formula. In case of engines, r=bore and h=stroke.

Originally Posted by Dr.AD

First of all, "imperfect" is not the right word for what you are trying to say. You probably meant not a round number. There is no such thing as an imperfect number. "Imperfect" almost sounds like something went wrong or someone missed something or someone is trying to pull of a fraud. None of that is the case.

This is true for almost anything. When an engine's power is rated at 140kW (as an example), it will not necessarily be an exact round number of 140.00. When a mobile phone's battery is rated at 3000mAh (again as an example), most likely that number is an approximation and a rounded up/down figure from the true number which will be slightly different from precise 3000.00. Even fuel tank capacities are an approximate numbers. Fuel tanks shapes are very complex, and if one has to design a tank with exactly 50.00 liters of capacity, it will require a lot of additional (and unnecessary) design work with zero advantage in return. How does it matter if the real capacity is actually 49.95l or 50.05l and then rounded to 50l

Yes, "imperfect" might not be the perfect term when considered in isolation, but I only used the term to present the context.

It is true that a vehicle's engine specification or phone's battery rating or for that matter anything is not usually a round figure. This a well know fact for which I am trying to know why. It is in human nature to aim at a round figure numerically in various aspects of life which might not be always achievable for various reasons.

Quoting an example for purpose of discussion and clarifying my point, it is a common practice that many of us go to the fuel station and fill 1000/2000 bucks worth of fuel. While doing so, we end up filling XX.xx litres of fuel. Or we generally opt for 10/20 litres of fuel, for which we end up paying XX.xx bucks. It is probably impossible to have both fuel the quantity and the amount to be in round figures. Atleast one of the variables can be constrained depending on the choice. This example of mine doesn't involve engineering for sure, but it is not so in an automobile engine.

Agreed, displacement alone is not a factor that contributes to the power output. What I am trying to understand is the approach that a manufacturer adopts while designing an automotive engine.

[Dr.AD]

Originally Posted by Emvi

It is true that a vehicle's engine specification or phone's battery rating or for that matter anything is not usually a round figure. This a well know fact for which I am trying to know why.

This is already answered in the above few posts. The reason is that there is no benefit in achieving a perfect round figure. Engine design is mathematically an optimization exercise where several parameters are simultaneously varied while meeting several design contrarians. While doing this optimization exercise, and "roughly" targeting the size say as 2l, it may happen that the optimum design is at 1.98l. There is no benefit in throwing away this design and try to re-optimize everything to reach exact 2.00l. This will involve a lot of additional design efforts for no benefits at all.

Remember than the math involved in engine design is not "integer mathematics". It is mathematics with "Real numbers" (which have fractions), and the output of real number operations is again a real number. There is no reason to expect the output to be an integer number. Leave alone the integer case, in real number operations, even the 1.5 may not be precisely 1.5. It may be 1.49999 or 1.500001 or something like that. There will always be some round-off errors. Again, technically it is possible for achieve the end result as precise 1.50000, but then it is not worth it. And then the question is, precise to how many decimal points? Who decides that? What is the benefit in all that additional work to control the decimal points which are practically not significant?

Quoting an example for purpose of discussion and clarifying my point, it is a common practice that many of us go to the fuel station and fill 1000/2000 bucks worth of fuel. While doing so, we end up filling XX.xx litres of fuel.

It is not an appropriate example. In fact, this is the opposite example. You round it to Rs. 1000 because that is convenient to pay. And there is no additional effort required to fill Rs. 1000 fuel because it can be directly set on the dispensing machine. Rounding up the bills while payments/purchase is not the same as achieving round figures as the output of an engineering design project. The latter requires a lot of additional work than the former, with no benefit to show.

[veedub89]

As far as the power figures go, I feel it's very arbitrary for the majority of the buying public.
What does 105HP really mean?
Is it crank horsepower or the power at the wheels? What does a car with 105HP at the crank feel like? How much better or worse is it than a car with 100HP at the wheels?

I feel these numbers only appeal to a handful of enthusiasts like us. And we have our own reference points with the cars that we drive currently to effectively gauge another car with a different horsepower rating than ours.

As a manufacturer, Rolls Royce never discloses its horsepower rating. Apple didn't use to declare how big its battery is in mAh.

So manufacturers are under no compulsion to have near "perfect" horsepower ratings that say are all divisible by 5 to achieve the "perfection". For most people these are just numbers on a page.

[Emvi]

Originally Posted by Dr.AD

It is not an appropriate example. In fact, this is the opposite example. You round it to Rs. 1000 because that is convenient to pay. And there is no additional effort required to fill Rs. 1000 fuel because it can be directly set on the dispensing machine. Rounding up the bills while payments/purchase is not the same as achieving round figures as the output of an engineering design project. The latter requires a lot of additional work than the former, with no benefit to show.

Well, it would have not been possible to call the example inappropriate without catching the context, therefore I safely guess you have. My intention was just that. Examples sometimes have little relevance to the subject but certainly help in clarifying the context. The examples you have quoted definitely have substance in it and your insights are well taken

[Reinhard]

To begin with - many times the engineers are designing engines to an "upper cap". Especially in countries like India where there are regulations and taxation slabs tightly coupled with engine displacements.

Bikes - <100CC, <125CC, <150CC and so on.
Cars - <1200CC, <1500CC and so on.

The moment there is a "<" sign and not "<=" - the engineers are dictated to keep the displacement BELOW this number. So 99.9CC is the max you can achieve in what is a 100CC engine per se.

On top of it - come actual engineering design parameters. To optimize the efficiency of parts, costs, mileage, power output, size and associated margins - the best design might be 99.3CC in a 100CC design bracket. I feel thats the drift in general, to put it very simply at a superficial level. Remember - its all finally down to money and wafer thin margins in slabs. So the cost of making even a 99.4CC engine might be staggeringly high compared to that of a 99.3CC engine if the safety/reliability thresholds of the materials and specifications are on the bubble.

The nomenclature of course is a bit generalized. For example - the venerable 1.3L MJD engine is just 1248CC. Mathematically it should be called 1.25L .

[Jeroen]

Bit of an anorak question. do we know of any (car) engine that are actually larger then what they are marketed for.

Say a 1.5l actually being 1.502 or similar. I seem to recall reading about that a long time ago.

Jeroen

[InControl]

Originally Posted by Jeroen

Bit of an anorak question. do we know of any (car) engine that are actually larger then what they are marketed for.

Say a 1.5l actually being 1.502 or similar. I seem to recall reading about that a long time ago.

Jeroen

Umm, not sure about cars, the entry level bike Bajaj Platina 100 has actually a 102 cc engine.

[slamnos]

The displacement of the engine to the last number is not decided by the engineers or designers.

To start with it is purely for classification or to cluster with competition (to make it obvious to the buyers).

The bore x stroke ratio is designed keeping in mind the objective to be achieved from the engine (power, torque, fuel efficiency, power delivery & even correlation to other related components like valve setup, firing angle etc to name a few).

The ratio is further fine tuned / altered based on the final objective that needs to be achieved. In due course if the engine achieves say 1197 cc against the badged 1.2L it is purely for categorisation.

[Emvi]

Originally Posted by Jeroen

Bit of an anorak question. do we know of any (car) engine that are actually larger then what they are marketed for.

Say a 1.5l actually being 1.502 or similar. I seem to recall reading about that a long time ago.

I can remember two such car engines.

1. Mahindra Bolero - previous generation.
2.5L engine which actually was 2523cc

2. Tata Tiago's 1.05L revotorq engine.
I am not sure if it was classified as 1L engine

I believe none of these are in production now.

[PearlJam]

Originally Posted by Reinhard

The nomenclature of course is a bit generalized. For example - the venerable 1.3L MJD engine is just 1248CC. Mathematically it should be called 1.25L .

Agree with you here. When the adage goes "there's no replacement for displacement", then it makes sense to always say the precise number 1248cc (or 1.248L) instead of 1.3L, at all places. Otherwise, a layman will never know this unless they dig deeper into the specifications. In this specific example, it's a relatively big 4.1% misrepresentation of facts!