Why turbos make for superior street and race engines


 by Mark Cipolloni
June 20, 2003

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In making a case for why CART should consider keeping a turbocharged engine formula, I recently went out on a limb to predict that in 10 years time, it's quite conceivable that 100% of all street cars will be supercharged or turbocharged.  If CART wanted to be in line with the passenger car industry, than clearly a turbo formula was the way to go.  While not quite that high an actual market penetration, this article explains in more detail why a turbocharged engine is superior to a normally aspirated engine, be it for a street car or a race car.

After doing some more research, I found that market analyst Global Insight, Martec, DRI and PSR forecasts the following penetration of turbocharged engines on gasoline passenger cars by 2010:

  • Worldwide increase from current 5% to 15-25% in 2010. Worldwide gasoline passenger car production is about 40 million cars/year, so 15-25% is a significant number of cars, far more market penetration than a V-10.

  • Europe increase from current 10% to 30% in 2010

  • Asia increase from current 4% to 6% in 2010

  • USA increase from current <1% to 5% in 2010

Superchargers, while better than normally aspirated , are projected to drop from 1.1% worldwide to about 0.5% by 2010 mainly because they do not give the required fuel efficiency improvement needed in the future. They will continue to be used in niche performance models.

The drivers for this growth in the use of smaller  turbocharged engines (turbocharged to make up for lost HP when reducing displacement) are:

  • Europe: The need to downsize to meet future CO2 emissions; fuel economy.  A smaller displacement engine makes less pollution and gets better fuel economy than a larger one, everything else being equal.

  • Asia: Government incentives; increased demand for smaller cars.

  • USA: Reduction in C.A.F.E. (Corporate Average Fuel Economy).

2010 is only 7 years from now, and my prediction was in 10 years a 100% market penetration might be reached.  Taking a more realistic view, 50% by 2013 is certainly not inconceivable once manufacturers fully embrace turbos, and there is no reason they won't as long as the gasoline internal combustion is mainstream.

The current single turbo should be changed to twin turbos, one on each side of the engine ala F1 years ago, and ALMS now

Based on what I have seen at LeMans, ALMS and World Rally Car (WRC) rules, I believe CART should give consideration to twin turbo intercooled gasoline engines with air inlet restrictors and maximum boost control as currently used by Audi, Bentley, Peugeot (and formerly Toyota, Cadillac and Nissan) at LeMans and by all WRC cars (sans the boost control).  CART would use higher RPM, larger air inlet restrictors, and higher boost, but reduce the engine displacement to somewhere in the vicinity of 1.5 L.

This combination offers the following advantages:

  1. Incredible durability. Turbocharged racing engines offer the possibility to achieve equal performance with significantly lower internal inertia loading because of smaller and, therefore, lighter reciprocating components operating at lower speeds. This can be achieved without the need for high cylinder counts or exotic and costly low-density super materials to reduce the reciprocating mass per cylinder in order to reduce the maximum connecting rod tensile load.  The different engine configurations bias one or two of the three independent variables that determine maximum airflow:
    • Displacement Volume
    • Engine Speed (RPM)
    • Air inlet Density (boost).
    Choosing an engine configuration with a bias for achieving maximum airflow through boosted charge density minimizes the need for either large displacement (i.e. higher reciprocating mass) or high engine speed, which has an exponential effect on reciprocating load. Put simply, components of smaller reciprocating mass operating at lower piston speed for fewer load reversal cycles is the secret to the dramatic durability performance demonstrated by turbocharged engines at the 24 Hours of LeMans in recent years. The Audi factory has entered the LeMans 24 Hours race 10 times since 1999 and has never failed to finish. Ditto for Bentley 5 times. Audi has run engines at as much as two ALMS races (2hr 45min each) after completing the 24 hours, without disassembly!  Audi has run at least two cars in the ALMS for 4 seasons and has never experienced engine failure.  This year in CART we see great reliability from the Ford Cosworth turbo engine.

  2. Cost. Because the turbo helps the engine to generate a lot more HP, the Audi runs at a maximum of 6500 RPM (CART would run higher boost pressure and turn them around 12,000 - 14,000 because the pistons will be a lot smaller so they could,  and they need the higher RPM to make it sound a bit racier.)  Because of the low RPM in the Audi,  there are no exotic materials needed for reciprocating components. No need for more cylinders to increase displacement without increasing reciprocating weight per cylinder. No need for pneumatic valves or even exotic valve springs. Long life between rebuilds. These engines aren't working that hard compared to competitive larger displacement engines (more reciprocating mass) that have to run much higher RPM to pump the same air as a boosted engine.  And if you are a passenger car manufacturer, don't you want to manufacture a 100,000 mile engine (i.e. an endurance engine) that also happens to make a bucket load of HP when called upon?

  3. Fan perception. If you are a fan, you should hate electronic rev limiters. I can't accept it when I am watching an in-car camera shot of a car pulling out of the draft to make a pass and Ch-Ch-Ch-Ch-Ch..... Every time I see it, it brings up images of the governor on the lawn mower I used to use as a kid. Unacceptable. An air inlet restrictor does the exact same job but the fans never know it because they can't hear it. It is beyond me why this isn't a huge technology image issue. You have one RPM limiter that is obvious and one that is silent. Which is better for image?   Maximum flow capability of the air inlet restrictor limits engine RPM.  Increasing RPM further above restrictor choke results in power loss, so is not possible. Restrictor size, not electronics, can be used to directly set maximum operating RPM.

  4. Technology image. We know from the BAIN report that CART had done a a couple of years ago, all levels of ChampCar fans listed technology as the #1 reason they buy a ticket or tune in to a ChampCar race.

    The Audi twin turbo intercooled gasoline installation with air inlet restrictors and boost control

    If gasoline turbo with intercoolers is used, this is really hard to do with the existing bell housing designed for a single turbo - no space for the intercooler. The solution is to do what is done at LeMans and was formerly done in F1, put a turbo and an intercooler on each side of the engine. Getting the turbo out of the hot box between the engine and transmission will be better for transmission and turbocharger reliability. Then when the engine cover is removed, the fans can actually see the intercoolers and turbochargers oozing testosterone. Actually being able to see technology must reinforce the image as opposed to just believing it is there.

  5. Noise. Somebody should take a sound decibel meter to Miami and measure the back-to-back noise levels of the Audi compared to the Judd and Panoz N.A. engines in ALMS and compare to Cosworth in ChampCar. Audi is by far the quietest, followed by Cosworth. The N.A. engines can cause hearing damage. If noise levels were monitored in CART over the years, it would be seen that the noise levels went up significantly as the boost was decreased (less exhaust energy routed through the turbo) and has dropped in 2003 because the boost was increased and there are no more single-sided turbo systems. The turbos act as an effective muffling device as long is it is not excessively short-circuited by wastegating of exhaust energy. It is beyond me why this isn't a huge issue with the move to more street races. The only way to make an N.A. engine quiet like a high-boost turbo is to install mufflers. See #3 above.

  6. Longevity. The formula can last practically forever with reductions in boost limit (low end) and restrictor diameter (top end). Just start big with both.

It seems to me those are six pretty important advantages that twin turbo intercooled gasoline engines can deliver over N.A. engines. And it leaves the sanctioning body in direct control of power via maximum RPM (restrictor diameter) and torque (boost limit) and in indirect control of durability and cost.

Even if CART goes with N.A. engines, they should use sonic rather than electronic RPM limiters just for image. Let the low-tech series have the low-tech image.  In the past the then four engine manufacturers would not accept air inlet restrictors because it limited their creativity to chase RPM increases and challenge their engineers. All of them are currently using electronic rev limiters (governors) now, so I don't think it is a problem for any of them any more.

There is a perception disadvantage if air inlet restrictors are used at Fontana or any track where racing is full throttle approximately 100% of the time. There they will be called "restrictor plates" and will create "restrictor plate racing" a la Daytona and Talladega NASCAR.  However, on road/street courses, air inlet restrictors leave plenty of challenge for engineers of different engine manufacturers because there are opportunities to optimize low speed torque and response that don't exist on super speedways because they are not needed for that type of racing.

By the way, WRC engines are limited to 6500-7000 RPM also by their air inlet restrictors. Even though they do not have boost limits, these engines are quite reliable at these engine speeds. Engine failures are rare, and all the engines must be production based.

So there you have it Chris Pook.  You said you wanted CART to be a vehicle for which passenger car manufacturers can sell cars.  I submit to you, and to CART, that unless you have some sort of deal with Bernie Ecclestone to use V-10 F1 engines in CART, it may behoove you to think twice about abandoning a turbo formula for 2005 and beyond, for all the reasons listed above.

Ref: Durability aspects of turbocharged vs. normally aspirated racing engines, by Doug Milliken

The author can be contacted at markc@autoracing1.com

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