When it comes to designing a high-performance sportscar or supercar, a mid-engine arrangement offers the ultimate chassis performance in terms of weight distribution, handling, braking and dynamic response. For this reason, most racing cars switched to the mid-engine layout in the 1960s; some, such as the famous Auto Union ‘silver arrows’ used the format as far back as the 1930s.
However, although positioning the powerplant in the middle of the car, directly behind the occupants, is best practice for optimising driving dynamics, such a design presents engineering challenges for road-going vehicles in terms of packaging, noise, vibration and harshness (NVH), and heat management. For the exhaust system, these issues must be addressed without compromising engine performance, exhaust emissions or sound characteristics – all essential factors for a sportscar.
In its recent collaboration with General Motors on the exhaust system for the new high-performance, mid-engine Corvette, Tenneco had to tackle all these challenges. The company employed several technical solutions to accommodate the packaging and thermal requirements of the emissions system while optimising efficiency and retaining the characteristic exhaust note that is synonymous with the V8 Corvette.
This was achieved by using a combination of active and passive exhaust valve technology, and advanced heat shielding techniques. The valves vary system backpressure and shape the exhaust sound to provide the right balance of performance, sound characteristics and regulatory compliance, while the heat shielding protects vulnerable items from excessive temperatures within the tight packaging constraints.
The lure of the mid-engine Corvette design
With so many hurdles to overcome, one might question why a manufacturer would go to the trouble of engineering a mid-engine installation, especially when it has a heritage of charismatic front engine/rear drive cars, such as the previous Corvette models.
The choice of engine location plays a very important role in the dynamics of the vehicle, particularly the weight distribution and the overall moment of inertia. For everyday utilitarian vehicles, such as city cars and family hatchback models, the preferred solution is a front engine with front-wheel drive, which leaves the bulk of the space available for passengers and their luggage. For high-performance sportscars, a more focused approach sacrifices some of this convenience to deliver a more engaging driving experience and accepts the impact on passenger space, cooling system layout and general packaging efficiency.
Placing the heaviest single assembly, the engine, close to the centre of the car makes it easier to achieve a more even weight distribution across the front and rear wheels, giving more neutral handling, particularly during enthusiastic cornering on undulating roads. Because centralising the mass of the vehicle reduces its polar moment of inertia, the car responds faster to driver inputs without resorting to steering geometry that could feel ‘edgy’ or ‘nervous’ during everyday driving. Sharing the weight more equally between the four corners of the vehicle also reduces the trade-offs in suspension tuning between handling and ride quality. For a road-going sportscar, all these characteristics combine to provide a more thrilling, yet safer and more comfortable driving experience.
Compared to a front engine/rear drive arrangement, the mid-engine layout improves traction under hard acceleration and benefits stability under braking and when changing direction. Today’s electronic systems such as ABS and skid control have less work to do, so can perform better, increasing safety margins. Moving the engine to a central position also frees up useful space at the front of the vehicle which can be used for the energy-absorbing crumple zones that make the vehicle safer in a frontal impact.
Disadvantages of mid-engine design
The compelling benefits of a mid-engine design must, inevitably, be paid for in other ways. The main disadvantage is reduced passenger space, which explains why most mid-engined vehicles are two-seaters. The second issue to arise is cooling; radiators must either be front-mounted and connected via suitable pipework, or mid-mounted with large ducts and vents to ensure adequate airflow. Either of these options can be incorporated readily into a sportscar as part of the overall styling but much less so in a more practical vehicle.
Mid-mounted engines present a greater NVH challenge than front engines because their intakes are much closer to the vehicle occupants’ ears. The audible soundtrack can become a plus point for a sportscar but would be considered intrusive on a more mundane vehicle.
Greater driver skill is necessary to extract the most from a mid-engine chassis because the low inertia that makes the vehicle so responsive can, at the limit of adhesion, make the car rotate more quickly. This makes control and recovery more challenging and could lead to a spin. Though electronic aids can eliminate this issue nowadays, the manufacturer must calibrate the system carefully to avoid overly intrusive intervention that would blunt the inherent driving appeal of the mid-engine layout.
One of the biggest challenges is engineering the exhaust system. Compared to a front engine installation, the available length is much reduced yet the same level of aftertreatment and silencing is required to meet exhaust emissions legislation and drive-by noise requirements. This must be packaged without impairing the engine performance or generating excessive heat in the engine compartment. At the same time, a sportscar exhaust must also produce an appealing and distinctive sound; quite a tall order.
The 2020 Corvette mid-engine system
Today’s high-performance exhaust systems use active valves to vary the system back pressure, producing the optimum balance of performance, sound characteristics and regulatory compliance. Tenneco Clean Air supplies two systems for the 2020 Corvette featuring either two or four electronic exhaust valves. The small block V8 LT2 engine with the standard exhaust develops 490bhp and 465lb-ft of torque; with the performance exhaust these figures increase to 495bhp and 470lb-ft, enough to propel the car from 0-60 in under three seconds, according to GM.
The standard exhaust has two newly developed valves with enhanced heat resistance that work with the vehicle’s Active Fuel Management system, supporting the engine’s cylinder deactivation function for optimum efficiency and lower emissions. The optional performance exhaust uses two additional electronically controlled valves at the rear of exhaust system, where temperatures are lower, to deliver acoustic differentiation. The driver can select ‘stealth mode’ for cruising, ‘sport mode’ for more dynamic driving or ‘track mode’ for the full Corvette experience.
The exhaust gas temperatures in the 2020 Corvette far exceed the thermal limits of conventional coupled electronic valves, as Dmitri Konson, vice president, global engineering, Tenneco Clean Air, explains: “Typically, such electronic valves cannot operate reliably above 750°C exhaust gas temperatures. This isn’t an issue in the vast amount of applications, but in the scenario of a mid-engine, high-performance configuration the exhaust gas temperatures can peak higher than the acceptable limit for conventional valves with electronic actuators.”
Tenneco’s solution was to develop a valve arrangement with the actuator decoupled from the valve body, operating through a link arm, to greatly reduce heat conduction to the valve electronics. This arrangement represents a considerable step forward in actuated valve design, and when combined with Tenneco’s latest tribological developments in valve materials, enables their use on mid-engine vehicles and others with extremely high exhaust temperatures.
The signature V8 sound that epitomises the character of the Corvette was faithfully preserved in the new model as a result of co-operation between GM and Tenneco. The partnership resulted in a new approach to flow mixing between the left and right sections of the cold-end exhaust system, achieving optimum acoustics across the entire RPM and load range. A low back pressure design with four tailpipe outlets enabled GM to meet its engine power and emissions targets, with and without cylinder deactivation.
They also addressed how to limit the unwanted transfer of high exhaust temperatures to the vehicle’s surrounding components. To effectively manage the thermal and packaging requirements of a mid-engine configuration, Tenneco’s engineers developed special multi-layer heat shields, constructed from a sandwich of insulation between two thin stainless-steel layers. The use of the multi-layer shield technology minimises the weight and package size of the finished assembly, providing the minimum profile to meet both the engineering and styling requirements.
The emissions system was a global effort led by Tenneco Clean Air’s Michigan, USA and Germany product and engineering teams. It is produced at the firm’s manufacturing facility in Tennessee, USA then shipped to the GM plant in New York, USA for final assembly. Some 1,500 people work at the facility, producing this and also others ranging from the two-litre Malibu unit up to the small block V8 for the latest Silverado and Sierra.