|
Both Citroen and Ford (that I know of) are offering Diesel engines with "Overboost". Any views on their effect on the reliability/durability of the engines ?
|
Overboost? What dat?
Sounds a bit 'Ripspeedy' - in an XtremePowerMax kind of way.
|
|
|
|
The term overboost is a confusing one. It would be better if it was called "more than average boost" as it just applies to allowing the turbo to give the engine more boost than say the level required to achieve 120 BHP from that particular engine. It is the wat that all ECU or chip upgrades work on turbo engines. The term has been hijaked by the marketing people to sound like something a bit daring and sexy ! Ultimately most car manufacturers produce a standard engine and a slightly more powerfull engine based upon more boost. Ther are some slight exceptions to this, for example Vag 1.9 TDi engines which have slightly beefier internal components for the 130 and 150 BHP variants as compared to the 100 and 105 BHP units.
|
|
As mentioned above, its a marketing term really. Overboost is there the ECU allows the turbo to briefly increase boost pressure for a short period of time under certain circumstances - the idea is to give brief extra punch (for example when overtaking).
|
I interpret it slightly differently:
That the ecu / engine will provide additional torque (not so much BHP*) when it is really needed i.e underload and depending certain perameters such as throttle position etc, this so it gives a real slug of torque when it is needed and not when it is not needed which improves the drivability, and perceived refinement. A slug of torque at an inapropriate time can can make a car feel unruly, i.e. pulling out of a side turning in first as opposed to pulling past an arctic at 45 in fourth.
(*Max BHP is a factor or torque and revs so is not effected by overboost because the increase in boost is a factor at lower revs, higher boost is available at higher as a matter of course. Though the BHP produced at lower revs will increase in line with the torque increase.)
|
">pulling past an arctic at 45 in fourth.<"
Now that is a big overtake. Did it include Spitzbergen? ;-)
|
When an engine has "overboost" I'd like to know whether the "normal" output for power/torque are quoted as maxima or the "overboost" output.
If the "overboost" output is quoted, which I suspect, then in reality the engine is being artificially throttled back under normal conditions. If this is the case reliability/durability should be improved because the engine can't be driven at "full" throttle for very long!
|
There is really no concept of 'full throttle' with these engines.
In a Diesel at cruising speed you have an excess of air, some EGR gas, and fuel being injected. The car is being 'throttled' on fuel supply. When you push the pedal, extra fuel is injected up to the point where all of the available oxygen in the air is being used - the torque output of the engine at this point to what is known as Zero Boost Torque (ZBT). To get more torque out of the engine the amount of air must be increased by increasing the boost - this is easy to do with a VNT because the ECU will dial in the extra boost via the rack. Boost can continue to be increased in this way. Obviously the loads on the engine increase and so durability will suffer if sustained high boost is used.
Basically the manufacturer determines the optimum normal boost for power/durability/economy. Anything above this is deemed to be 'overboost' and can be allowed for short periods under certain circumstances (e.g. full throttle with engine at 2000rpm). Overboost does not work at low engine speeds because the turbo can't provide the necessary extra boost pressure.
I suspect that max power outputs are quoted under overboost - but this is not the maximum sustainable power output for the engine though.
Whether you like the driving characteristics of all of this is up for debate. Personally I don't like driving with an engine where everything happens between 1800-4500rpm, overboost or not.
|
>> throttle with engine at 2000rpm). Overboost does not work at low engine speeds because the turbo can't provide the necessary extra boost pressure.
A great explanation Aprilia though I beg to differ on one point, on a TD overboost does work at low speeds, the idea is that it delivers torque when appropriate, i.e underload and depending certain perameters such as clutch position, throttle position, road speed etc, so it gives extra torque when it is needed and not when it is not needed as per my post above. Take a 2.2 TDCI / X-Type max torque 360/400nm @ 1800rpm without/with overboost. Even if overboost is not called upon at low revs then max boost will still be achieved at higher revs, i.e. overboost does not effect max power.
It is really a marketing term to cover the ability of the ECU to provide additional torque when it is required and not when it is not required.
|
|
Though I suppose it does depend on what you mean by low revs, if you are meaning much below 1800 then no of course the turbo cannot deliver the boost required.
|
Though I suppose it does depend on what you mean by low revs if you are meaning much below 1800 then no of course the turbo cannot deliver the boost required.
Yes, I mean low revs as in below about 1800 or so. At low rpm the turbo is not getting enough energy to overboost. Overboost is really effective in the mid-rpm, high-throttle-opening regime.
|
|
I would prefer an `underboost` option so reducing the potential for squandering fuel with an inadvertant heavy foot. :)
|
|
If it's anything like the overboost function fitted to the Porsche 911 Turbo, then it only activates at full throttle, increasing the boost from the turbo for around 10 seconds, increasing the torque.
|
If it's anything like the overboost function fitted to the Porsche 911 Turbo then it only activates at full throttle increasing the boost from the turbo for around 10 seconds increasing the torque.
Yes, same idea. Its by no means new. Older turbo experts will remember a version of it when it was called 'boost spiking'.
|
|
