sions after-treatment challenges limit this as an option for reducing fuel consumption.
Optimize timing of spark event—an important factor since this affects the countervailing variables of in-cylinder heat loss and thermodynamic losses. This is discussed in more detail below.
Maximum efficiency occurs when the two countervailing variables, heat loss and thermodynamic losses, sum to a minimum. The optimum spark timing is often referred to as minimum advance for best torque or maximum brake torque (MBT). At low to moderate speeds and medium to high loads, SI engines tend to be knock-prone, and sparktiming retardation is used to suppress the knock tendency. Spark-timing adjustments are also made to enable rapid-response idle load control to compensate for such things as AC compressor engagement. For this to be effective, idle spark timing must be substantially retarded from MBT. Retardation from MBT for either of the aforementioned reasons compromises fuel consumption.
Gas exchange or pumping losses, in the simplest terms, refer to the pressure-gradient-induced forces across the piston crown that oppose normal piston travel during the exhaust and intake strokes. The pumping loss that principally affects fuel consumption is that which occurs during the intake stroke when the cylinder pressure and the intake manifold are approximately equal. The pumping loss component that occurs during the exhaust stroke mainly affects peak power. Both of these oppose the desired work production of the engine cycle and thus are seen as internal parasitic losses, which compromise fuel efficiency.
The main source of friction losses within an SI engine are the piston and crankshaft-bearing assemblies. The majority of the piston-assembly friction comes from the ring-cylinder interface. The oil-control ring applies force against the cylinder liner during all four strokes while the compression rings only apply minor spring force but are gas-pressure loaded. Piston-assembly friction is rather complex as it constantly undergoes transitions from hydrodynamic to boundary-layer friction. Hydrodynamic piston-assembly friction predominates in the mid-stroke region while boundary-layer friction is common near the top center. Avoidance of cylinder out-of-roundness can contribute to the minimization of piston-ring-related friction. Crankshaft-bearing friction, while significant, is predominately hydrodynamic and is relatively predictable.
Engine architecture refers to the overall design of the engine, generally in terms of number of cylinders and cylinder displacement. The engine architecture can affect efficiency mainly through bore-stroke ratio effects and balance-shaft requirements.
Trends in power train packaging and power-to-weight ratios have led in-line engines to have under-square bore-stroke ratios (i.e., less than unity) while most V-configuration engines have over-square ratios. Under-square ratios tend to be favored for their high thermodynamic efficiency. This is due to the surface-area-to-volume ratio of the combustion chamber; under-square designs tend to exhibit less heat transfer and have shorter burn intervals. Over-square designs enable larger valve flow areas normalized to displacement and therefore favor power density. These interactive factors play a role in determining overall vehicle fuel efficiency.
Balance-shafts are used to satisfy vibration concerns. These balance shafts add parasitic losses, weight, and rotational inertia, and therefore have an effect on vehicle fuel efficiency. I4 engines having displacement of roughly 1.8 L or more require balance shafts to cancel the second-order shake forces. These are two counter-rotating balance shafts running at twice crankshaft speed. The 90° V6 engines typically require a single, first-order balance shaft to cancel a rotating couple. The 60° V6 and 90° V8 engines need no balance shafts. Small-displacement I3 engines have received development attention from many vehicle manufacturers. These require a single first-order balance shaft to negate a rotating couple. While low-speed high-load operation of small displacement I3 engines tends to be objectionable from a noise, vibration, and harshness (NVH) perspective, they could be seen as candidate engines for vehicles such as hybrid-electric vehicles (HEVs) where some of the objectionable operating modes could be avoided.
Parasitic losses in and around the engine typically involve oil and coolant pumps, power steering, alternator, and balance shafts. These impose power demands and therefore affect fuel consumption. Many vehicle manufacturers have given much attention to replacing the mechanical drives for the first three of these with electric drives. Most agree that electrification of the power steering provides a measurable fuel consumption benefit under typical driving conditions. Fuel consumption benefit associated with the electrification of oil or coolant pumps is much less clear. Electrification of these functions provides control flexibility but at a lower efficiency. Claims have been made that the coolant pump can be inactive during the cold-start and warm-up period; however, consideration must be given to such things as gasket failure, bore or valve seat distortion, etc. These factors result from