Engine RPM vs Cruising Speeds Part 3

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!!