Life Cycle of an Automobile Engine

Preamble
I’m often asked what’d be the estimated life of a car engine, say an M800 or for that matter even a Honda City. Needless to say I find myself at a loss for words when pitted against such ones. Sample the following QnA that had with a fairly knowledgeable guy…

Question
What’s the funda for an ICE’s life i.e. how much should I expect a used Honda 1.3 DX engine to last - given that it shows 42 Kkm on the odo as of now. I have heard that 1-Grand Mark is the near end of a Car engine and time for an overhaul. Would that mean I only have another 58 Kkm or so to see before I have to visit the garage for an expensive overhaul? Or does it also depend on the way one drives and maintains the engine??

Answer
A question like this is difficult to answer with reasonable accuracy. It’s some what like asking why some Guys croak at 30 and some > 75, when the average life expectancy in India is 50!

For example, I have seen any # of M800s in DLH with engines killed by 70 kkm whereas I had a ‘91 std with a Retrofit AC, driven by atleast 6-members of the family over a period of 10 yrs/144 Kkm, with engine still in full pep/nil oil consumption – to the extent none could figure out that the odo had once turned over!

For starters, consider the following extract from my Article ‘Funda’s of Automobile Engg.’

Q: What is Engine Life Factor (ELF)? How can it be calculated?

A: It’s a ‘Factor’ given by the Formula ‘ELF = 100,000/Max RPM x Compression Ratio’ of an ICE. Since it’s a ‘number’ only, it’s devised to ‘compare’ the Life and Reliability amongst comparable ICEs.

Now where does that leave one - gasping for breath or waiting to exhale!?

On the other hand, it’s been my experience, based on funda’s of A-E, that the life of a Car Engine largely depends on the following - perhaps in that order:

1) Its basic design/materials, ‘workmanship’, and OEM-QC standards.
2) How well mated it’s to the Car by way of ‘Power to Weight Ratio’ - anything below 70 is an ‘edge’ design.
3) The proverbial ‘nut behind the wheel’.
4) How well it’s been ‘run-in’ and subsequently driven/serviced/oil-changed.
5) How often and for how long it’s been driven at >75% of its max rating - rpm/bhp. Cars in India lose out on this front compared to their western counterparts that log most of their mileage in 5th gear a/a ours in 3rd - typically translating to 1.66:1 ELF wise!

It’s my reckoning that OE Engines of most present day cars with PWRs > 70 can easily do 200 Kkm+ if ‘well looked after’. However, when it comes to assessing the ‘residual life’ of a used car engine, one can go about it scientifically as follows:

A typical expression found in various W/S Manuals in respect of the ‘health’ of a 9:1 Compression Ratio (87 Octane) Petrol Engine states:

i) Max ‘dry’ CR - 13.5 kg/cm @ 400 rpm/normal Starter cranking speed.
ii) Permissible Inter-cyl variation - (+/-) 1.0 kg/cm.
iii) Wear-down/service limit - 10.0 kg/cm.

Therefore, by implication, one can predict the remaining useful life of an engine by comparing its present CR with its values under (i) and (iii) above, vis a vis its ‘true’ Odo reading at the time of such a Test.

However, this will tell you about the state of its pistons/rings and valves only and not about its various other wear prone parts, such as journals/bearings etc. Only a ‘trained’ ear can tell the latter to some extent, by listening to its ‘noise spectrum’ at various loads/speeds.