Kevin Hoag

Engine Bearing Design

Within any given engine a large number of bearings incorporating several design, material, and operational variations are seen. While needle, ball, or roller bearings are sometimes seen in automotive and heavy-duty engines the majority of engines for these applications use primarily plain bearings. Roller bearings are receiving increasing attention due to their lower oil supply requirements and potential for reduced friction and parasitic losses in automotive engines. Increased space requirements, and in the case of connecting rod bearings increased rotating mass, must be weighed against potential attractions. Automotive engine examples of ball and roller bearing are seen in valvetrain and balancer shaft bearings. The discussion presented in the following sections will be limited only to plain bearings.

Camshafts and the Valve Train

The poppet valve, as previously detailed in Fig. 9.5, is now used universally in four-stroke vehicular engines—both to draw fresh charge into the cylinder, and to exhaust the spent products. The valves face an especially harsh environment. Because they are exposed directly to the combustion chamber, and provide very restrictive heat transfer paths they operate at especially high temperatures. The demand for rapid opening and closing results in high impact loads, and a requirement for high hardness valves and seats. The combination of high hardness and high temperature requirements drives the selection of special steel alloys, typically with high nickel content for both the valve head and the valve seat. Most automobile valves are made as a single piece, while the valves in heavy-duty engines generally have the nickel alloy head inertia welded to a mild steel stem. Hollow stem two-piece valves are generating interest in automobile applications, for savings of both weight and cost. A valve spring and retainer assembly as shown in Fig. 9.5 completes the installation. The retainer is typically stamped from mild steel, and holds the spring in a partially compressed position with two hardened steel keepers fitted near the top of the valve stem.

Pistons and Rings

The piston remains one of the most challenging components to successfully design, and is certainly quite critical to the performance and durability of the engine. The root of the challenge lies in the piston’s role as the moving combustion chamber wall. It is thus directly exposed to the severe conditions of the combustion chamber, and must manage the work transfers between the combustion gases and the connecting rod. Further challenges implied by this role include the necessity of maintaining a combustion seal at this moving boundary, under a wide variety of operating conditions; the need to provide adequate lubrication and minimal friction and wear at an elevated temperature, and under continually starting and stopping conditions; and the requirement of managing the reciprocating forces as well as secondary forces resulting from the pivoting motion at its pin to the connecting rod.

The Engine Development Process

The intent of this chapter is to provide an overview of the processes involved in developing a new engine from the initial need identification to production release. A flowchart of the processes has been developed by AVL List GmbH, and is presented in the Figure. For ease of reference, the paragraph headings in this chapter correspond to encircled numbers in the figure.

Block and Head Development

The concepts of reliability and durability were previously introduced in Chap. 3. That chapter was introduced with the example of the cylinder head to emphasize the importance of ensuring the durability of such major structural components. In this chapter discussion returns to the cylinder block and head, applying the concepts of Chap. 3 to the durability validation of these components. Both the cylinder block and cylinder head are expected to perform without failure for the engine’s life to overhaul. In automobiles the expectation is extended to include reuse in at least one engine overhaul and in heavy-duty applications to several overhauls. This expectation translates into one of an extremely low failure rate (less than one in many thousands) after many miles or hours of operation. If this expectation is not met the engine quickly gains a bad reputation from which it is extremely difficult to recover. This is especially devastating when one considers the investment in tooling for a new engine. A battery of tests and analysis must be devised to absolutely ensure that these durability expectations are met—an extremely challenging endeavor, and a critical path in the timeline of engine development.

Cranktrain (Crankshafts, Connecting Rods, and Flywheel)

The Cranktrain is at the heart of the reciprocating piston engine, and its purpose is to translate the linear motion of the pistons into rotary motion for the purpose of extracting useful work. The cranktrain is typically composed of connecting rods, the crankshaft, and a flywheel or power takeoff device.

Cylinder Head Layout Design

Several of the decisions discussed in previous chapters determine the starting point for cylinder head layout. In Chap. 6 the trade-offs determining cylinder bore were presented, as were those that determined the number of cylinders making up a bank. In Chap. 8 the variables that go into determining cylinder spacing were discussed, and the number and approximate placement of the head bolts was introduced. The question of camshaft placement was also introduced in Chap. 8, and will be taken up again later in this chapter.

Engine Configuration and Balance

Once the fuel type, engine operating cycle, total displacement, and supercharging decisions have been made for a new engine, the next tasks will be to decide upon the number of cylinders over which the displacement will be divided, and the orientation of the cylinders. The factors that must be considered include cost and complexity, reciprocating mass and required engine speed, surface-to-volume ratio, pumping losses, packaging, and the balancing of mechanical forces. The majority of this chapter will address the mechanical forces and engine balancing as this is a key factor explaining why particular numbers and orientations of cylinders are repeatedly chosen.

The Internal Combustion Engine—An Introduction

It is appropriate to begin with a simple definition of the engine as a device for converting energy into useful work. The goal of any engine is to convert energy from some other form into “mechanical force and motion.” The terms “mechanical force” and “motion” are chosen to convey the idea that the interest may be both in work output—how much force can be applied to move something a given distance—and also power output—how quickly the work can be done.

Cylinder Block Layout and Design Decisions

In Chap. 5 the required engine displacement was calculated, and in Chap. 6 the number of cylinders, cylinder layout, and bore-to-stroke ratio were determined. These earlier decisions form the starting point for the discussion of this chapter. It is in this chapter more than any other that it will be necessary to limit the discussion to designs specific to automotive applications. Over the entire range of reciprocating piston internal combustion engines there is an extremely wide range of cylinder configurations and block layout and construction techniques. By limiting the discussion to automobile engines and heavy-duty engines in mobile installations, primary attention will be placed on in-line four, five and six cylinder engines, and vee six, eight, ten, and twelve cylinder engines. Casting the net a bit wider allows discussion of horizontally opposed four, six, and eight cylinder engines, and mention of the recently revived ‘W-8’ and ‘W-12’. The cylinder block is the foundation of the engine, and supports the piston, cranktrain, cylinder head, and sometimes the valvetrain. It also houses the lubrication and cooling systems. It provides mounting points for the charging system, starting system, power take off (PTO), and typically has mounts which support the entire powertrain. The engine may be rigidly mounted as a structural member of the chassis, such as in a racecar or motorcycle. The cylinder block supports a variety of static, dynamic, and thermal loads, and must provide stiffness and alignment for many components. Because of the complexity of geometry, and complexity of loading, hand calculations are rarely used. Simplified finite element analysis (FEA) of a single power cylinder is usually the starting point, prior to analysis of the entire assembly.