Some QnA’s on Life Cycle of an ICE

Preamble
As is not unusual, the last week’s Article on the Life Cycle of an Automobile Engine evoked some pertinent questions from one of the regular e-readers of this column. I won’t be surprised if most of you had similar reactions. So this is how it went…

Question
Just went through your article on ‘Engine life’ and was curious about the line ‘Maximum dry CR 13.5 kg/cm @ 400 rpm’. I assume ‘dry’ means cranking the engine without allowing ignition to take place and ‘CR’ means compression ratio, but what does the figure 13.5 kg/cm stand for? Is it pressure (i.e. kg/sq. cm)? VG/Mumbai

Answer
‘Compression’ Tests are of 2-types - i) ‘Dry’ meaning as done below and - ii) ‘Wet’ meaning after injecting a 10-20 ml of engine oil thro’ the spark plug opening ‘before’ cranking the engine for the test. Ideally speaking, in a healthy engine, there shldn’t be much difference between the two. However, if the piston/rings are worn out, there will be significant difference. So a ‘wet’ test is a sort of double check before concluding that the p/r’s are gone.

It’s a std procedure when taking the ‘compression’ readings to - i) remove all the spark plugs/Air Filters, ii) depress the Accl Pedal fully and iii) crank the engine by the starter motor, which is usually 400 rpm per typical petrol engine starter motor design, till the pressure gauge screwed in place of a spark plug/cyl registers a max reading (2-fellows needed). The PG used is a ‘Stay-Put’ type, like a Tyre Pressure Gauge, which permits one to read it accurately and then ‘reset’ to zero.

The fig ‘13.5′ is - i) the ‘pressure’, either in psi or kg/cmsq here, recorded by the PG used as above and ii) ‘13.5′ is the fig for a healthy engine having a compression ratio of 9:1 - as per Suzuki Work Shop Manuals.

Question
Also, would the pressure not depend on the compression ratio, as well as the inlet pressure (in supercharged/turbocharged engines, this would be above atmospheric).

Answer
Obviously it’ll be higher for engines having ‘Compression Ratio’ higher than 9:1. At 400 rpm, atleast a Turbo Charger will have no effect coz the engine is prevented from firing by removal of its Spark Plugs and in a Super-Charged engine, the pressure build-up will be negligible at such a low rpm. May be they recommend taking off the S-C drive belt in such a case but I won’t know off hand as I’ve yet to come across a work shop manual an S-C car!

Question
Wouldn’t it also not depend on the condition of the air filter and the inlet/exhaust valves (more the fouling, less the pressure)?

Answer
As I’ve said above, the air filter is removed for such a test. I suppose valve/seat wear goes hand in hand with piston/rings wear, as far as an engine performance is concerned.

Subsequent Responses
Thanks for the very precise answers to my queries! It has improved my understanding of automobile engines. I remember I had the chance to witness a ‘dry’ test, after the garage had done a ‘carbon clean’ chemical process on my Maruti-800 a few years back.

However, instead of the ’stay-put type’ pressure gauge you mentioned, they had an instrument that took the compression readings on ‘cards’, much like an engine indicator (used in marine engines). He claimed that the improvement in readings was due to the ‘carbon cleaning’ of the valves/valve seats and pistons/rings. He even inserted a sort of probe inside the spark plug hole, to show me on a video screen the cleaned insides - an instrument similar to what ENT Doctors use to look inside one’s ears/throat! VG.

That was an interesting experience you shared with me VG. However, this ‘Carbon-Clean’ using machines for the purpose can be dicey and do more harm than good, as certain minimum carbon deposits at ‘strategic’ locations within an engine are actually necessary, to enable it achieve its highest possible/designed compression ratio. SKG.