This statement is so unconditionally open ended that you can’t possibly answer it without specific qualifications and stipulations as there are a HUGE number of variables.WhiteBlue wrote:If I read you right you deny that the turbocharged engine will be intrinsically more fuel efficient than a NA engine. I find that hard to believe.
Is this simply a comparison of two similar capacity and architecture units with only the method of induction being the differentiation?
What is the turbo unit’s primary purpose?
Is it a performance engine or an economy engine?
Are there fuel flow restrictions in place?
What were the priorities of the engineers?
How is the engine “used” in the comparison?
Does the engine require increased fueling for in cylinder cooling when “on boost”?
What intercooling systems are applied to the intake?
Also, as far as I can see you haven’t actually defined what your definition of fuel efficient is.
Is it the typical fuel used per unit of distance traveled maybe you are looking at specific output per unit of fuel used or are you trying to equate specific output by capacity or perhaps the efficiency of the energy recapture systems?
From personal experience in racing turbocharged vehicles, it is not uncommon for a smaller capacity turbo engine to use more fuel than a larger capacity NA engine at the same race. Turbos while extracting more power from smaller capacities require substantial fueling to produce that power and remain reliable for various reasons.
All that aside, and assuming that we are discussing two similar capacity and architecture units running on the same fuel, I would on first principles say that a turbo charged engine will be intrinsically less fuel efficient (that being fuel used per unit of distance traveled) than a similar naturally aspirated engine.
However, it will provide similar power and possibly better fuel efficiency than a dissimilar larger capacity engine when used under certain circumstances.
More simply a turbocharger is simply a torque multiplier which works by increasing the effective capacity of the engine to which it is attached. It does this by increasing the absolute intake pressures above ambient atmospheric pressures, increasing the volume of oxygen available in the intake charge which allows more fuel to be utilised for power production.
Where a turbo charged engine is utilizing the extra intake pressure, there is a corresponding increase in specific fuel consumption and a corresponding increase in specific power output. As such a turbocharged engine will (generally) produce more power than a similarly size NA engine, however it does so at the cost of a proportional increase in fuel consumption reducing fuel efficiency.
As such, a turbocharged engine can be less fuel efficient when measured as fuel used per unit of distance traveled.
So, Edis’ statement is correct in saying that, generally “The reason you turbocharge a gasoline engine isn't for efficiency, …..instead you use turbocharging to compensate for the power loss caused by downsizing.”
IMHO, what Edis is saying (please correct me if I am wrong) is that turbochargers are used to increase the specific power output of smaller engines when “on boost” to regain any power deficit from the smaller capacity and allow the advantage of the “off-boost” fuel economy gains from the smaller engine when using it in cruise or low load situations.
Edis is also correct that a smaller capacity engines generally also benefit from lower pumping, frictional, inertial and heat losses than a larger capacity engines. However this discounts the effect of typically lower compression ratio’s on off-boost performance and fuel economy which is another consideration.
I think you are drawing a few very long bows in your conclusion and making a few assumptions without looking at the wider view.WhiteBlue wrote:If you go back to the example of the Porsche Cayenne V8 engine you will find a discrepancy with your statement. Porsche did not downsize the engine but added a turbocharger in order to raise the power.
Yes, Porsche did not downsize the 4.8Lt V8 engine. This decision had nothing to do with an increase in power for the Turbo and Turbo S and this decision had absolutely nothing to do with fuel efficiency and everything to do with economies of scale for the production of engine units and the associated cost savings.
As such, fuel efficiency for the Turbo and Turbo S was negatively affected, however in very simplistic terms power increased over the NA engine’s 294kw’s to the Turbos 368kw’s & the Turbo S’s 405kw’s.
So if you are simply equating efficiency to power per unit of capacity of the engine then sure, it’s simple maths.
294kw / 4.8Lt = 61.25kw/Lt is lower than
368kw / 4.8Lt = 76.67kw/Lt which is also lower than
405kw / 4.8Lt = 84.38kw/Lt.
However, as a family member actually owns a current series Porsche Cayenne Turbo S, which I use as frequently as I can, I have some experience with its fuel economy or lack thereof!!
The 4.8Lt V8 NA Cayenne GTS has official combined fuel economy figures 10.7L/100km (21.98 MPG)and it’s VERY easy to get it up around 18 or 19Lt/100km (12 - 13 MPG) in city or "spirited" driving. You really need to push the NA engine to get it moving.
The Cayenne Turbo S lists 11.5Lt/100KM (20.45MPG) as its official combined fuel economy figure which is as a starting point worse than the NA GTS, so it is intrinsically less fuel efficient per unit of distance traveled. I can also 100% assure you that if you push the Turbo S and use the boost from the turbos, it is more like 25+lt/100kms (9.5 MPG)!!
Moving away from the simple fuel consumption figures, you must also consider that in adding the turbos and increasing the maximum output of the engine, they also increased the overall weight by 150kgs (2,140kgs vs. 2,290kgs) by the addition of turbos, manifolds, intercoolers, associated plumbing, larger brakes, stronger components etc. which negatively effects fuel efficiency and the engine unit simply weigh more and if you are seriously reviewing a units efficiency then you must consider it resulting weight.
Perhaps more importantly, the engine itself also saw increases the fuel injector sizes and most importantly a reduction in the compression ratio from 12.5:1 to 10.5:1 which reduces off boost efficiency negatively effecting fuel efficiency. You cannot simply exclude the increased weight associated with turbo charging nor the off boost loss of efficiency from reduced compression ratios in any efficiency calculation IMHO.
So are you actually advocating that the Cayenne Turbo S is more fuel efficient than the Cayenne Turbo and the Cayenne GTS given its published fuel consumption data??
I would find that hard to believe and the numbers certainly do not support your theory, nor does my wallet and its experience with the Turbo S’s fuel consumption @ US$2.30/Lt or US$8.72/gallon
WhiteBlue wrote:If you look at the figures they suggest that the turbocharged version is considerably more fuel efficient than the naturally aspired version.
As above I do not see it in the published figures, so what are the figures of which you speak? As per the above and from my personal experience, the any Cayenne Turbo S is definitively NOT more fuel efficient than the NA version.
Another example would be the Subaru Impreza and STi. Both are similar versions of the same chassis with similar architecture engines, one being turbo and the other NA. The STi is certainly less fuel efficient from a fuel used per unit of distance traveled however it does produce more power from the same capacity.
Standing aside from the fact the source has a very obvious vested interest it certain statements, you also need to consider the context of the comments.WhiteBlue wrote:This is also what the Garrett expert whose words opened this thread suggest for a road going engine. He said that you turbocharge for better driving experience and fuel efficiency.
What I think is being said is that you turbocharge a smaller capacity engine to regain some of the power lost in reducing the capacity and gain an efficiency advantage against a larger capacity engines by utilizing the “off-boost” fuel economy gains of the smaller capacity for cruising/low load situations. I do not think he was advocating turbocharged engines as more fuel efficient specific performance or race platforms.
I would proffer that he is not saying turbocharging a similar engine makes it more fuel efficient than a naturally aspirated version, especially so in performance applications, but that it can make more specific power and be advantageous against a larger capacity engine thanks to other factors like weight and low load cruise.
F1 turbos were renowned for their extreme levels of fuel use and the previous turbo era is often referred to as the “gas guzzling turbo age”. Hardly a resounding endorsement of their fuel efficiency.WhiteBlue wrote:If we look at the last time we had turbos going against NA engines in F1 it certainly looked like the turbos were more fuel efficient than the NAs.
1987 saw 1.5Lt turbos limited to 4 bar absolute boost pressure, no fuel limits running against 3.5Lt capacity NA engines. You can’t possibly be advocating that the 1.5Lt turbos used less fuel than the NA 3.5Lt capacity engines.
1988 saw the 1.5Lt turbos limited to 2.5 bar absolute boost pressure and fuel limits of 155lts/race running against 3.5Lt NA capacity engines. Again, the turbos dominated however they had constant problems meeting the fuel requirements where boost was often turned down where they were passed by the NA runners. Once the boost was turned up again they could catch the NA drivers but the turbo cars barely finished the races with remaining while from all reports the NA runners were starting with less than the maximum 155Lt to gain a weight advantage. Again, this shows that the turbo engine were not more fuel efficient, but that they were less efficient and required careful monitoring of their fuel use so as to even make the end of a race.
Agreed, it is more efficient on first principles in the capture of potentially lost energy, however you are still only recapturing energy (primarily as heat) provided by the burning of extra fuel and that energy isn’t directly correlated to an equivalent increase in power of the engine. It is transformed through various stages and still relies on the use of extra fuel so is not a direct correlation.WhiteBlue wrote:There is also the indisputable fact that adding a turbocharger will reduce the kinetic and thermal energy level of the exhaust gas at the tail pipe. All other things being equal that necessitates a higher efficiency of the turbocharged engine. The turbo engine can convert that energy difference into useful power that is wasted by the NA engine.
You said that “all other things being equal”, however very little is equal in this comparison and there are a myriad of variables to consider. Lower compression ratios, increased weight, increased fuel consumption to match increased air flow, exhaust restrictions, different cam profiles etc, the list is very, very long. You need to be careful not to confuse fuel efficiency with efficiency of energy recapture which can result in the use of more fuel in simple turbo applications.
Also as an aside, the kinetic energy of the exhaust stream is secondary to the primary heat energy extracted through expansion in the turbine housing.
I would put it forward that were the fuel flow restrictions not put in place in 2014, there would be zero efficiency increase coming from the move from the current V8’s to the smaller turbo V6’s. The turbo V6’s would use similar or even greater amount of fuel to cover the same race distances as engineers would simply increase boost to make equivalent or greater power and use corresponding amounts of fuel.WhiteBlue wrote:I'm not saying that downsizing profits only from turbo charging. There is also the aspect of the improved mechanical efficiency that you describe. Both effects are contributing to the success of downsized engines. It would be wrong IMO to deny any of the two effects their contribution to the efficiency improvement.
As such, fuel efficiency, which again should not be confused to efficiency of energy recapture, is simply equitable (in my mind at least) to the amount of fuel used to cover a specific unit of distance. Turbo's simply do not generally provide an increase in fuel efficiency for most (not all) applications and certainly not for performance or racing applications.
