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
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.
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 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. Channel D could support a microphone input if chosen.
• Crankshaft rotation frequency is calculated via a mongoose serial interface, with channel ABC via the NVH 3 channel hardware.
• The vertical scope trace reports mass energy as milli g (gravity).
• The horizontal scope trace is separated by the various frequencies of vehicle operation.
The catch is with the environment in which the test is concluded. Road vibration, throttle position, load, braking and steering will affect your results.
I conducted several drive cycles, each with differing drive conditions, and I have chosen exerts from various traces in presenting my evidence. My first shows smooth combustion events. Please refer to Fig.1, the first of three Pico screenshots. When mass (g) is shown in 3D and rotated, you can see each combustion event within the record length. It should be even in peak value and profile.
Following careful selection of events throughout the trace shown by the shaded section of the record period the following anomalies were discovered. Please refer to Fig.2. The trace clearly shows omission of combustion contribution. I would draw your attention to the transfer of energy to the crankshaft E1 when combustion is unequal, resulting in the DMF having to absorb the anomalies. Now, please refer to Fig.3. The evidence appears to point to injector delivery imbalance, so prompting the only and correct action; Removal and a full injector bench test.
For the injector report, please refer to Fig.4. The injectors were supplied via a nationwide discount chain, and had been refurbished by a diesel manufacturer. As a result of the comprehensive defects discovered, our future injector suppliers will be a local specialist.
In conclusion the decision was taken to replace the injectors for genuine new units. Peter, our senior tech, commented that combustion noise reduced and engine performance dramatically improved.
However, returning to work I was presented with comments from our customer that vibration was still present. We now arrive at the ultimate diagnostic option, customer opinion. The game continues! I promise to update via my topics if we progress further.