[Samarth 619]

Originally Posted by CarML

Thank you for this very informative thread. What an apt example of the old Tata truck!
It would be interesting to understand the following as well.
1. How are gears used to best leverage the parameters to provide optimal performance for a given car?
2. How do engineers tune the same engine to produce different power figures? What are some of the judicious decisions taken while doing so?
3. How does the equation change dynamically for hybrid vehicles?
Thank you again.

1. When it comes to gears, the important thing is ratios of each gear, obtained by number of tooth in the driving gear/ the driven gear, within the gearbox.
So, now we have 5 forward ratios.
High numerical values mean undergearing, high acceleration, high roll on performance in general, high rpm's all the time.
Lower numerical values mean high gearing, means you cruise well, and get higher speeds in the gear per rpm. This usually means high fuel mileage too, as your engine travels higher but with low rpms/fuel consumption.

Ideally, your lowest gear should have enough drive for uphill, high load situations, and highest gear should be able to sustain a reasonable speed, and cruise effortlessly.
One choice is to space the gears:- lower gears too low, higher gears geared too high. But this makes gears too spacy, a single sihft causes a massive difference.
Hero Karizma (motorcycle) was a good example of irregular gearing. It was derived from a Honda CRF 230 which had 6 gears, but the Indian Karizma got 5:- three gears too low, fourth and fifth gear were quite high geared. So, when you shifted from third to fourth, the rpm drop was noticeable.


2. A lot of things exist in tuning world.
Using 2 values per cylinder (1 inlet, 1 exhaust) makes more low rpm torque, using 4 valves per cylinder (2 in, 2 out) gives more top end power.
Using Variable valve timing/actuation gives you best of both worlds: more torque at low rpm and more power at high rpm. Yamaha YZF R15 uses this, and in your car world, I suppose Honda City got it at some time, am I right?
Using SOHC gives lower power, using DOHC made higher power. DOHC, Double overhead camshafts are able to manage valves better than just one.
The Compression Ratio of your engine, the higher it is, the more power & torque it makes, but it also makes heat so it needs liquid cooling, plus this huge compression makes it tough to kick start or push start the bike/car so self start becomes compulsory. Lower CR means easy kick starts and more reliable, cooler engine.Altering the Air Fuel Ratio is also used by some tuners, not by companies though, coz its an unreliable, unscientific method, which also affects pollution.
But a higher flow air filter, combined with additional fuel input, might help a bit to increase performance reliably, for companies too.

Yamaha at one time promoted a blue core engine: the spark plug had a blue flame. No idea if it helped in fuel efficiency as they claimed for FZ16, but it was better at fuel mileage (60~kmpl) than previous bike (40-45kmpl~).
Bajaj keeps multiple spark plugs in its single cylinder engine:- apparently the fuel burns better and it gives a little more performance and efficiency. I think it helped because by using triple spark in Pulsar 200 NS, they kept its exhaust very lightweight and economical to buy/replace, it was just Rs.3000 and weighed under 10 kgs.

Wider bore results in higher performance overall due to wider valves and more fuel burnt. Longer stroke gives more torque per rpm. But both these changes change the engine itself:- the cubic capacity (cc) of the engine is changed.
Today, mostly short stroke and big bore engines are preferred, due to higher performance and power, in the motorcycle world. Some engines are rebored for bigger output. At one time, the entire Pulsar range was rebored versions of an another Pulsar engine, whether it was 150, 180, 200 or 220.
Supercharging and turbo charging puts your engine in another league:- it gains huge performance for any cc range. Example is Kawasaki H2.

Mostly these changes help a lot.

3. Hybrid have mostly electric drives and fuel based engine drives combined. Electric engines give a lot of torque right from 0 rpm, their torque curve is a straight line upto 70% of rpm, after which it starts reducing mostly.
The way electric cars pick up on signals, you'll think it has a 4 liter engine inside. But its just low rpm, later on the petrol picks up.
So clearly, a hybrid can benefit in mostly 2 ways: giving you useful electric assisted torque when you speed down, but don't downshift the gear. Second being, drive in electric mode most of the time, and use petrol for high speed/rpm performance, for economy reasons.
Some supercars use both modes to provide earth shattering performance, that is also an (expensive?) approach.

A lot depends on the software that decides which engine will be used when:- or the driving modes that you choose.
Maruti's cheapest micro hybrids are somewhat of a sham- they use starter motor as drive motor in weak situations, but it helps with mileage a little. Govt had also registered 2 cases against them, no idea what happened in them.

Originally Posted by VCheng

Thank you for that good explanation.
Over a lifetime of buying and enjoying cars and bikes, I generally have found that engines with nearly equal (or comparable) figures for both horsepower and torque (the more the better) tend to suit my driving style and usage the best.
Engines that tend to produce much more of one compared to the other tend to be more specialized.

This approach could get confusing because we measure power in kw as well as bhp, and we measure torque in NMs & Foot-Pounds, and maybe more units exist than what I mentioned.

Try to focus on the rpm where the maximum torque is made. Being around this point means you're likely to have overtaking force at your throttle, and you'll also be unaffected by heavy load, headwinds, tailwinds, uphill climbs, etc.For example, at one time, Suzuki sold two 125cc bikes, Zeus and Heat, and they made 10NM torque at just 3500 rpm.
It was mighty fun puttering around in the city on this bike, coz you were always at the max rpm zone. People could confuse it for 150cc bike going by performance, unless they rev it hard.

[VCheng]

Originally Posted by Samarth 619

This approach could get confusing because we measure power in kw as well as bhp, and we measure torque in NMs & Foot-Pounds, and maybe more units exist than what I mentioned.

Good point about the units, but as a Flatus Ancientis, my comment assumed good old bhp and ft-lbs as the units for power and torque.

(I also measure engine capacities in cubic inches, distances in miles, and fuel in gallons, you know, the real units. )

[Jeroen]

Originally Posted by VCheng

Good point about the units, but as a Flatus Ancientis, my comment assumed good old bhp and ft-lbs as the units for power and torque.

(I also measure engine capacities in cubic inches, distances in miles, and fuel in gallons, you know, the real units. )

Here is a question for you then:

Which miles are you using (there are at least 4-5 different definitions of miles around!!). Similar for gallons. There are at least three different measure of the gallon that I know off, the internet might know more!

It was not that long ago that different definitions of the inch were common too.

Whereas what you would like to consider real might be in the eye of the beholder, unless you standardise it, it will always cause massive confusion.

Major engineering disaster have happened because engineers did not use the same unit definition. More than once, people have died over incorrect conversion mistakes. Totally avoidable. But for some reasons, people feel cool and are often proud adhering to old, obsolete and often incorrect or ambivalent definitions of units.

https://www.mentalfloss.com/article/...sion-disasters

I won’t comment on the main topic of this thread. Unless you are Neil deGrasse Tyson, coming up with good analogies to explain an engineering principle, is simply way beyond most of us.

However, using the correct unit with a standard definition goes a long way. I have seen a lot of folks using incorrect units and mixing various units in error.

If we go by the SI system:

Force in Newton (N)
Length/distance in meter (m)
Power in Watt (W)
Time in seconds (s)
Energy in joule (j)

Torque is a rotating force (N) applied at a certain distance (m) perpendicular to the point of rotation. Moment is identical, but is a static force. Both have the same dimension Nm

Note Nm does not a have plural. 1 Nm, 100Nm etc.

A Watt is the amount of torque per time unit, so Nm/s

One joule is equivalent to the energy used to move a body by using one newton of force over the displacement of one metre. So 1 Nm is equal to 1 joule. So a Watt is also equal to j/s.

If you ever use more complex (engineering) formulas it is good practice to check the unit dimension per parameter. The only way that works (without having to use endless conversion factors) is using SI unit.

We need a Sutripta on this thread!

Jeroen