Word on the street is that the balance shaft on the diesel is causing a noise. This could be a bearing issue or oil pressure not getting to where it needs to be? Who knows, very little info has been released.. in true bmw style lol
while you can do all the bench testing in all the world, you must remember very little real world testing is ever done.
many development engines are put in dyno cells and fed fuel and air mixtures based on various altitudes and ambient temperatures.. with the right test cells this can be all done on the bench.
im sure mini did test the engine packaged up in the actual car.. but may have missed the noise due to many reasons. Development cars are not manufactured in the same way as a production vehicle... in the most part the production lines arent yet in place - so many assumptions are generally made at these early stages.
Therefore when the engine was loaded into the real production cars, engineers may have experienced (what we call in the game) excessive NVH - noise, vibrations & harshness...
There are many harmonics a diesel engine transmits, to the point that noise could be more apparent in the mini that what it might have been in the bmw 1 series... each package offers so many variables - some of these things get thru to the customer.
you can be sure anything safety related will not get thru in this way as those items are tested and tested again in simulated real world scenarios.
Back to the engine and why it needs a balance shaft...
in most cases engines, especially v engines, are typically smooth when running... this is taken care of by the crank design & the counter balance the crank would have built in - we call it the crank web.
we also use crank dampers pulleys to soften off NVH with the engine....
when you get inline engines you can only go so far with crank design and using damper pulleys... Thats when something like an I3 comes along.
An I3 engine is inherently out of balance due to its firing order... firing pulses has a tendency to induce an end-to end rocking motion... which makes the engine shake.
this shaking can be counteracted by using a balance shaft... a shaft driven by the crank with a weight attached. This zaps power from the engine and makes it less efficient than an engine not using a balance shaft...
take the ford eco-boot engine... now this I3 uses some pretty clever crank angles and an unbalanced flywheel to remove the need to use balancer shafts... cool eh!
To reduce noise the main timing chain / belt has been housed within a wet casing, which also helps to keep the noise down.
The major difference between diesel and petrol engines is the way these explosions happen. In a petrol engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a diesel engine, however, the air is compressed first, and then the fuel is injected. Because air heats up when it's compressed, the fuel ignites. The compression ratio is normally a lot higher in a diesel engine.
A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency.
The side effect of this higher compression ratio is the noise.
Diesel engines have in the past always been fuel injected where petrol engines didnt... mechanical injectors were very noisy...
common rail injection is a common thing these days between petrol and diesel engines... and very good system for controlling emissions.
Conn-rods, crank, & block material dont normally play that much of a part when it comes to balancing out an engine... there are various reasons as to why certain materials are selected, generally longevity, strength and weight.
over the years ford has made engines from various mixtures of ali / cast iron / steel... all ali, or all cast iron blocks...etc...
these days we are driven to make engines as light weight as possible, so ail is the material normally used... however, the most recent I3 petrol engine used CGI... when normally it has been ali in recent years...
hope this post didnt go too deep and explains enough to understand the differences....
i work at ford, in powertrain, if you didnt gather that already lol