Entries from December 2007 ↓

Preamble
Until late ‘90s, Power Steering per se’ was not provided by the carmakers as an OE fitment, at least in their entry-level hatch’s and sedan’s. But slowly things began to change when Hyundai first gave its option in their Santro.

Others naturally followed suit, with Maruti being the first to come out with ‘Electronic’ Power Steering in their smaller cars like the Alto and ‘Hydraulic’ ones in cars like Esteem and Baleno. So here’s an attempt to distinguish between the two systems and their pros and con’s…

A ‘Power-Steering’ is a ‘System’ which minimises the effort required to turn the Steering Wheel of a Car - which becomes more and more as the Cars get heavier and is worst during slow parking manoeuvres. Today they’re basically of two types – ‘Electronic’ and ‘Hydraulic’. Both eventually draw power from the engine - of the order of 3-5% - unlike power brakes.

The degree of ‘power-assist’ is designed to taper off as the car’s road speed increases, to maintain that ‘road feel’ at its steering and is, therefore, it’s designed to be max at standstill. It naturally comes at a price and then becomes another system in the Car to maintain - with attendant costs and vulnerability to failures - like any of its other systems.

An ‘EPS’ is relatively more ‘power-efficient’ than an ‘HPS’ and having lesser moving parts with no high-pressure hydraulics, it’s also less prone to breakdowns and maintenance. However, the flip side of it is that it requires a higher Capacity Battery/Alternator - especially for heavier Cars - thus indirectly adding to the overall cost of the System

On the other hand, an HPS runs off the engine directly. Eventually, both draw their power from the Engine only - which is the sole ‘prime mover on-board’. While HPS’ have been around since WW-II, EPS’ are < 10 yrs old and have yet to reach their full development, by way of providing that ‘feel of the road’ as compared to an EPS.

Here’s a real life QnA:

Question
For a car having low curb weight like Alto, Spark or Santro, is it wise to take a car without power steering? Does it affect the driving comfort significantly? Some people say that power steering makes a car more prone to accidents at high speeds.

Answer
For all practical purposes, cars tipping around 700-750 kgs can do w/o power steering. However, with increasing urban congestion leading to b2b parking and not to mention sissiness amongst men and women, power steering even for such cars is being looked upon more as a necessity than luxury.

As regards PS-Cars being prone to accidents at high speeds, it’s more of a myth since both the electronic and hydraulic ones are ’speed sensitive’, where the degree of ‘power assist’ is designed to go down with road speed. In fact the EPS guys claim that above 40 kph, the ‘degree of assist’ is zero i.e. revert to full manual.

On the other hand, with HPS, coz the system is always pressurised from both sides of the front/steering wheels, it infact resists sudden/involuntary direction changes at high speeds, to the extent that small unbalance in the steering wheels also doesn’t immediately result in a ‘wobble’, even at high speeds.

Preamble
The last two articles (Part 1 | Part 2) on the subject evoked some more, if not grilling, responses from my ‘regulars’.

Here is an interesting one from an Instructor level Marine Engineer – who has been around the world several times – if not ‘80’ - including on super-tankers as their chief engineer.

This is how the QnA went with him…

Question
Sorry to butt-in SKG, but I think the question was: What rpm would give the most mileage.
I don’t know that much about cars, but on ships, it is a simple rule: the faster you go, the more fuel you burn! Although the specific fuel consumption (gm of fuel burnt per unit power per unit time) may be better at higher rpm, the total consumption is directly proportional to the power consumed. The faster you go, the higher the resistance (its a cubic curve), consequently the more the wastage!

Even with my car (an Indigo-D), I’ve found that if I keep a steady speed of 80 kmph (5th gear) on the Expressway to Pune (the recommended speed for best specific fuel consumption), I get an average of 16 km/litre. If, however, I chug along at a sedate 55 - 60 kmph, I have got between 19.5 to 20 km/litre! Of course, it’s really boring driving like this !! VG/Mumbai.

Answer
You’re most welcome any time, as it invariably gives me some more food for thought!

Anyways, what you say is quite right - as it’s elementary physics. However, as far as I know, besides ‘rated’ rpm, there’s a basic difference between automotive, marine and stationary power plant engines - in the sense that the latter have their fuel efficiencies ‘optimised’ for ‘near’ full load coz that’s where they’re expected to operate ‘most’ times.

Whereas an automotive engine is required to operate at different rpm’s ‘all’ the time, for reasons one can very well imagine and appreciate. Therefore, it’s ’specific fuel consumption’ is optimised for an rpm/bhp somewhere around 50-60% of its max ratings, to give it a reasonably exciting cruising speed as well as make it run as relaxed as possible and yet have a sufficient ‘reserve power’ for contingencies.

Another notable difference between automotive and stationary ICE’s is their rated rpm. Since the former have to run at various speeds and yet have weight/volume constraints that come with mounting on a car, the present day trend is towards higher and higher rpm capability - both for petrol and diesel. Where as for the stationary ones, the lower is considered to be better coz it not only results in longer life/in hrs but also, most times they’re burning near asphalt like fuels - as you know very well - that require longer full-burn times.

That given and said, it follows that the speed at which an automotive engine will consume the least amount of fuel is the minimum speed at which it can ‘cruise’ w/o any ‘hiccups’. This, if you’ve observed, is of the order of 30+ kph in the 4th gear and 40+ in the 5th for most present generation cars. In other words, nearly ‘no-throttle’ for an Mpfi/Crdi car!

In fact, the ‘record breaking’ FC’s achieved by various cars like the Ford-1.4D, Logan-1.5D, Verna-D etc. that one keeps hearing about in the media, of the order of 30+ kpl, are achieved under such speeds only! But then, as you’ve yourself said, it’d be awfully boring to be driving on the EW’s at such speeds for hours together. In reality, at such low speeds, at least my adrenalin also drops so low that I tend to doze off at the wheel!!

Preamble
The last week’s article on the above subject evoked some interesting responses from some of my ‘regulars’.

Here’s one from a power systems engineer by ‘training’ (like me!) and this is how the QnA’s went with him…

Question
SKG, what is the rpm that is recommended for the best mileage? I tend to ‘cruise’ at 2K rpm in my Wagon-R - not that one can really ‘cruise’ in a city like Bangalore but I do because I leave home at 5.15/5.20 am for the golf course for teeing off at 5.45 am and so I can actually ‘cruise’ at that hour. I choose the 4th or 5th gear to be able to drive at 2K rpm. Is this OK or should I do a higher or lower rpm while cruising? – NJR/B’lore

Answer
It’s somewhat difficult to answer your question with respect to today’s Mpfi’s, where the engine ‘performance’ can be manipulated at the design stage to a very large extent, by programming its ‘ECU’ accordingly. In the pre-EU-II/carbureted cars of yore, it was believed that an engine was most fuel-efficient at its ‘peak-torque’ rpm - which was generally around 2500-3000 rpm and max/bhp rpm’s were of the order of 5-5,500.

However, to me it appears that this is no longer true. Take the example of a 1600cc car like a Baleno, where its peak torque occurs at 3500 rpm and in its Vth gear at this rpm, it’d do close to 140 kph! Whereas its ‘red-line’ happens to be around 6000 rpm - with a top speed ‘capability’ of 185 kph. The corresponding figs for a W-R, peak-torque and max-bhp rpm-wise, are quite similar to a Baleno’s. I remember my one such ‘92/M800 with an ingeniously designed ‘on-board’ Tacho used to do 80kph in IVth (no Vth gear then) at 3000 rpm.

It may be interesting to know that the present day tendencies of car designers are to have higher and higher peak torque rpm’s, to boost the accelerating capabilities of a car. Whereas for cars basically meant for pottering around town with minimal gear changes, such as a W-R, it’s desirable to have them around 3000 rpm - clearly a clash of interests.

To sum-up, I feel one can get the best out of a present day/petrol car - small or big - by limiting the highway cruise rpm’s to 3000-3500 from engine health/longevity points of view and to get the best ‘overall’ mileage, cruise between 80-90 kph in the Vth. The latter will correspond to 2000-2500 rpm for most petrol cars. For diesels, depending on their max torque and Bhp, these figs could be lower by, say, 500 rpm for the same road speeds.

Preamble
Early in October this year, I’d essayed on the fuel ‘efficiency’ vs fuel ‘consumption’ of a car, giving an example of an old Zen cruising at different speeds.

As a fallout of it, a friend posed me some further questions but this time, talking about the differences in engine rpm as displayed by the Tacho on Dash for different cars. This is how the QnA went…

Question
I was reading the other day an AutoExpress/UK review about a BMW-5 Series which does 100 mph @ 1700 rpm. My question to you is how do these guys make an engine perform at such low rpm’s at such high speeds, considering that my Verna-D though not comparable to the BMW, touches 100 kph only at around 2200 rpm in 5th.

So is there a specific logic to their engine/gearbox design that such cars work leisurely at much lower rpm’s whereas the rest are really pulling more than twice that number. I am foxed and hence this question to you. CM/Hyd.

Answer
Thanks for the teaser CM. After a long time some one has asked me an interesting question that sends my creative juices a-flow. Coming down to think of it, it’s not much of a brainer. So here we go:

We all know that an ICE basically produces a ‘torque’ i.e. a ‘rotary force’, like an electric motor and, therefore, has an Rpm vs Torque ‘Characteristic’. We also know that ‘torque’ is primarily responsible for the accelerating ‘capability’ of a car, of course in tandem with its gearbox/ratios.

Since ‘power/(b)hp’ i.e. ‘rate of doing work’ is a direct multiple of torque and rpm, it follows that - a) the bhp vs rpm ‘characteristic’ of an ICE closely follows the shape of its primary torque vs rpm characteristic and, b) that the ‘max power’ dictates the top-speed ‘capability’ of a car.

Now, the power required to move a car at ‘a’ speed basically depends on - a) its ‘road-rolling resistance’ and, b) the ‘wind resistance’ faced by it. At higher speeds, the latter is predominant and it goes up ‘exponentially’ with road speed.

Coming now to the root of your question. Assume for a moment that (a)+(b) above are the same for a Verna-D and a BMW-5D, for a road speed of 100 mph/160 kph, that a Verna is also quite capable of doing, AND that to move them at this speed, their respective engines must produce the same, say, 100 bhp.

Now comes the crux. Since the Verna can do only ~110 bhp/max, say at 4,000 rpm, to be doing 100 bhp for a 160 Kph speed, it’s engine will have to be spinning at least at 4000×100/110 = 3636 rpm.

On the other hand, the BMW-5 produces 190 bhp at 4,000 rpm. Therefore, for it to produce only 100 bhp to move @ 160 Kph, its engine stands run only at 4000×100/190 = 2105 rpm, or 58% of what the Verna has to do. And eureka, this is more or less what you’ve observed!

Please note that the actual bhp vs rpm figs in real life will vary, depending upon the ’shape’ of their individual bhp/rpm curves but for simplicity’s sake, I’ve taken them to be a straight line @ 45*.

Moral of the story, a high/max bhp engine doesn’t mean it can rocket a car to the moon BUT the ‘reserve’ power it has while cruising at an optimum or recommended road speed, so that - a) it cruises with the engine most ‘relaxed’ and more importantly, b) it has a tiger-in-waiting to literally ‘leap forward’ as and when required - either for over taking or just for the kicks!

It’s with this funda at the back that some of the current models of Rolls’ don’t have a conventional/rpm Tacho on the Dash. Instead, what they have is an rpm meter ‘calibrated’ in terms of ‘remaining’ engine power at a given rpm, showing ‘Percentage Reserve Power’ at any driving speed in any gear!

And coming down to think of it, not so long ago until the world at large started laying due or undue emphasis on max torque and max bhp even for high end cars as their USPs, ‘RR’ used to declare ‘Adequate’ against the Engine/bhp columns of its Specs!!