Efficient Performance Revisited

Efficient Performance Revisited: Combining Power with Economy

The past year has seen my become a very active member on the Automation Forums. One of the topics hosted was a challenge to build an engine that could return decent economy and still provide a decent amount of power. However, to balance the users' engines, all of them were limited to zero quality points on each component. Moreover, even with this restriction in place, they also had to be reliable; there would not be any point in building such an engine if they suffered too many problems over time. With this in mind, I set about building an engine that could meet all of these requirements.

The first step was choosing an engine configuration. I thought about using a V configuration, but eventually rejected it due to the increased friction losses inherent to those designs. That left me to choose between a straight-four or a straight-six. I picked the latter, and not just because it was inherently balanced (and hence smoother); it struck a delicate balance between affordability and output. Besides, considering the prevalence of turbocharged straight-fours in real life, I felt that building one of those would have smacked of conformism - seldom a good thing in today's increasingly homogenized automotive landscape.

Behold the ultimate in efficient affordable performance engines!

Speaking of turbos, I realized that it was actually the only way to achieve the desired level of efficiency - although I could get good results with a normally-aspirated engine, it could only take me so far before too many major compromises would have to be made. Also, the emphasis on economy forced me to use low-friction cast pistons, which were less durable even than ordinary cast pistons. Finally, having insisted on a redline of at least 7,000 rpm, I was stuck with a short-stroke engine, although I also envisaged a pure performance variant, and to that end I gave the engine provision for a longer stroke.

Low-friction pistons are a must when economy is the top priority.

Configuring the turbo installation was the toughest part of the build. In particular, the compressor size had to be carefully selected so that it would extract maximum efficiency without any discernible loss in performance, and this involved checking the efficiency chart, from which the fuel consumption trough (the lowest point on the chart) could be determined. With this information, I used a small compressor, but not one that was small enough to excessively restrict incoming airflow, and applied this idea when deciding the size of the turbine. In addition, I capped the boost pressure at 0.6 bar (8.7 psi) to allow for a relatively high compression ratio, thereby improving economy still further. I could even have reduced the AR ratio from its eventual value of 1.4, but opted not to do so, since I feared I would lose too much power in the process.

A 21st-century turbo setup that makes the most out of late-80s ball-bearing technology... in terms of balancing economy and performance.

Calibrating the fuel system was also crucial to achieving the desired economy rating. Direct injection (which allowed for even higher compression ratios and leaner air/fuel ratios compared to port injection) wouldn't be enough on its own. I had to cut the AFR to 15:1 (the leanest one possible) and advance the ignition timing considerably (specifically, to 84/100; any more advanced and the engine would be damaged or even destroyed by detonation, and even if it wasn't, it would most likely not have been as economical). All that was left to install an exhaust system of the right diameter, again to balance economy and power output.

An efficiency-oriented fuel system optimized to deliver decent performance was vital to this challenge.

The end result was an engine with an efficiency rating of 37.57%, but the quest didn't stop there. Having created an engine that competed for overall honors, I set about attempting to extract as much efficiency using an overhead-valve V8, which is inherently less efficient. This time, however, the simpler valvetrain made my task much easier; I just had to use a mild cam profile (but not so mild that efficiency would actually be reduced) and tweak the ignition timing to suit. With an efficiency rating of just 29.81 %, it was nowhere near as efficient as my turbo straight-six, but it would have made a nice entry-level V8 for general-purpose duty.

My personal best result with a naturally-aspirated overhead-valve V8 was nowhere near as impressive as the turbo six, but still very high for an engine of its type.

I learned a lot of useful stuff from this challenge. Specifically, it may take several hours to maximize the efficiency-to-performance ratio of a particular engine, but the reward is well worth the effort. This is most true when the engine is to be used in a car whose target market prioritizes economy. Also, when you are working on a tight budget, not using quality points at all can actually be beneficial to your car's competitiveness. And yet, after this, there are still many engine design challenges for us Automationeers to explore and learn about...