BY Frank Massey This month’s topic could easily have focused on dual mass flywheels (DMFs) as well as our chosen subject, combustion anomalies. The common thread here is of course vehicle vibration, using oscilloscope analysis using Pico’s noise vibration harshness (NVH) test kit. As a shampoo advert once observed; Here comes the science part – Concentrate! The dual mass flywheel was of course developed to store vibration (mass energy imbalance) and return it to the transmission. It achieves this principally with two large radial springs connecting the primary and secondary flywheel mass. Further development introduced small pendulum weights mounted around the outer circumference. The object was essentially threefold; Improving occupant comfort, reducing component weight and improving fuel efficiency. There are two edges to this sword however. Along with the various benefits it brings, which we just covered, the DMF will also absorb and mask a certain amount of vibration from genuine vehicle faults. The challenge technicians face is how to approach vibration issues, what tools and evidence to trust and the inevitable getting-it wrong accusation. Cause The vehicle in question, a Ford 1.8 tcdi has recently been fitted with a replacement DMF and complete set of refurbished injectors. The diagnostic evidence and symptoms were not known to me so can only comment on our customers most recent complaint of vibration. I was asked for a second opinion and to apply my NVH kit to establish cause. I began by simply sitting in the vehicle with a serial tool with the intention of checking for any serial evidence. Upon starting the vehicle, I experienced a substantial vibration which felt like a NVH issue. The vibration was most evident at idle with no interaction from the throttle. The obvious next check with a diesel engine imbalance is injector fuelling correction. Injectors 1/2/4 reported a value of 0.7. Injector 3 reported a value of 1.4. This was not a convincing result, so the NVH was my next choice. Before we explore further, I need to explain my views on combustion anomalies. I always avoid the expression misfire because it pre-focuses on combustion failure rather than combustion imbalance. The smooth running of all diesel engines is controlled by fuel quantity and timing. Allow me to introduce the NVH element to my theory on combustion. The efficiency of combustion is a measure of the energy applied to the piston crown, therefor can be expressed as mass. As such can therefore be evaluated 24 AFTERMARKET FEBRUARY 2024 TECHNICAL www.aftermarketonline.net A MONTH IN THE LIFE OF A VEHICLE TECHNICIAN Frank gets into double NVH trouble with a dual mass flywheel on a Ford 1.8 DCI with an accelerometer and the aid of the NVH software. The software provides the ability of separating the various frequencies when operating a vehicle: E1 crankshaft rotation, including DMF balance of rotating and reciprocating components. E0.5/E2 combustion energy events Transmission, input shaft, ley shaft, main shaft Road wheel frequencies, tyres, rim, drive shaft, differential Now for the fun! The magnetic base three -dimensional accelerometer is placed on the driver’s seat frame, scope channel A forward facing, B vertical facing, C lateral facing. Part ten Fig. 1 Fig. 1
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