The fuel economy of modern cars is amazing. Decade by decade, manufacturers have developed increasingly frugal vehicles, while meeting stricter controls on emissions and despite the weight increase driven by higher levels of equipment. To the consumer it seems there is no downside to this progress: the cars are still fun to drive, with ample performance and good levels of refinement, so what’s the problem?
The fact that most consumers cannot see any drawbacks associated with improved efficiency doesn’t mean there aren’t any: an army of engineers throughout the automotive industry has worked tirelessly to overcome the challenges involved. Ironically, the more successful they are, the more invisible their work becomes to the consumer: being taken for granted is the sign of a job well done.
Take a few examples. You want better fuel economy with the same performance? OK, they say: we can build a smaller engine, maybe with fewer cylinders or running more slowly, to cut friction then boost it with turbocharging to give the same power and torque as a bigger one. The drawback is that each cylinder now fires with a much bigger bang, sending unacceptable vibration through the powertrain and into the car.
How about reducing losses in the transmission, so less power is wasted on its way to the wheels? If we use thinner oil and run the gears on needle bearings we can achieve this. Unfortunately we now risk objectionable rattling from the gears, excited by those engine vibrations and unfettered by drag which previously damped out the effect.
Many cars now have an automatic stop-start function, to cut out idling when stationary which wastes a disproportionate amount of fuel in heavy traffic. Unfortunately, every time the engine restarts, additional demands are made on battery life and engine bearings which now have to withstand perhaps thirty times the starts of a conventional engine.
Most people have heard of turbo lag – the delay between putting your foot down and feeling the vehicle accelerate. With a highly turbocharged engine and a stop-start system, the challenge of making a prompt getaway from a standstill feel acceptable to the driver is considerable.
Lighter cars generally use less fuel, so improving efficiency drives manufacturers towards the use of lighter materials. Unfortunately these typically provide less inherent damping, so that rattling sound from your high efficiency gearbox is more likely to be transmitted to the cabin.
This catalogue of issues shows that the single-minded pursuit of improved efficiency could leave us with a noisy, highly strung vehicle that’s sluggish off the line and wears out faster. Thankfully, intense competition between manufacturers means such a product would never reach the market or, if it did, would soon disappear.
Engineers working for the vehicle manufacturers and tier one suppliers have developed a combination of new technologies and design techniques to enable consumers to enjoy the benefits of lower fuel consumption and CO2 emissions without the drawbacks.
Dual Mass Flywheels (DMFs) prevent the bigger bangs in today’s engines from transmitting unpleasant harshness to the transmission and the rest of the car. Multiple turbos, with large and small units on the same engine, or combined turbo and supercharger technologies, allow smaller engines to match the performance of larger ones without unacceptable lag in the throttle response. Special crankshaft bearings with increased wear-resistance provide the same service life with stop-start systems as their predecessors managed with normal engines.
Sophisticated analysis techniques allow lighter structures to match the noise levels of older, heavier designs by aiding the understanding of the different modes of vibration inherent in the various systems. The skill is in ‘tuning’ the response to eliminate unwanted characteristics or interactions that transmit or exacerbate noise, vibration and harshness (NVH). One of the greatest differentiators between competing brands is the level of attention to detail in this field.
It should then be no surprise that as emerging manufacturers in developing markets catch up to the established vehicle producers in terms of performance, functionality and styling, the last gap to be closed, the area in which they struggle most, is that of refinement. Producing a vehicle with competitive performance, economy, price and refinement requires the most thorough understanding of complex and interacting systems. It is also a clear indication of just how well developed current mainstream vehicles have become, because the engineering trade-offs ‘beneath the skin’ are largely invisible to the consumer.