Engine Building Fundamentals: Cylinder Head
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Cylinder Head
06.03
| 00:00 | - The cylinder head is the key to the airflow into the cylinders, as well as the exhaust flow out into the exhaust system, and as such it has a dramatic effect on the performance of the engine. |
| 00:12 | There are a wide variety of cylinder head designs, and I can't do justice to all of them in this module, however, we will cover the basic aspects of the cylinder head, as well as the most popular techniques for valve actuation. |
| 00:26 | The cylinder head is most commonly made from an aluminium casting, and bolts to the top of the engine block, and seals to the block thanks to the head gasket. |
| 00:36 | This gasket is actually responsible for sealing combustion pressure, the cooling system, and the oil supply and return ports that run between the block and the cylinder head. |
| 00:48 | The cylinder head will also have a water jacket cast into it that surrounds the combustion chamber, and provides coolant flow where it's required. |
| 00:58 | One of the key aspects to the cylinder head is the valves that control flow into and out of the cylinder. |
| 01:04 | The valves can't run directly on the aluminium material of the cylinder head, so some considerations need to be made here. |
| 01:12 | Firstly, a valve guide is fitted through the cylinder head for the valve to locate and operate in. |
| 01:19 | It's common for the valve guide to be made from cast iron in OE applications, while in performance engines, bronze or manganese bronze material is more popular. |
| 01:30 | The cylinder head will also have a hardened insert known as a valve seat that the valves actually close and seal against. |
| 01:38 | An aspect that's often overlooked is that the valves, and in particular the exhaust valves, rely on contacting the valve seat to transfer heat out of the head of the valve, and if the valve is not seating properly or it's held off the seat by debris, it may end up melting because it can't get rid of this heat fast enough. |
| 01:58 | If we move up in the cylinder head, we have the valve spring, which sits on a hardened seat in the cylinder head, and controls the valve movement. |
| 02:07 | Above the valve spring we'll then have a retainer, and this is attached to the valve with a pair of collets. |
| 02:15 | These collets locate in a collet or cotta groove machined near the top of the valve stem. |
| 02:21 | A valve stem seal is fitted to the top of the valve guide, and seals against the outside of the valve to prevent excessive oil consumption. |
| 02:31 | When it comes to valve actuation, there are a variety of techniques employed by engine manufacturers. |
| 02:38 | At the simple of these is the push rod engine, where the cam is located in the engine block, and the valves are actuated via push rods and a rocker mechanism. |
| 02:48 | With overhead cam shaft cylinder heads, where the cams are located in the cylinder head, the valves may be directly actuated by the cam lobes, or via a rocker or finger follower arrangement. |
| 03:02 | If the valves are directly actuated by the cam shaft, then typically a bucket will be fitted over the top of the valve, which serves two purposes. |
| 03:12 | Firstly, the bucket provides a wider surface area for the cam lobe to run on than if the lobe was to actuate directly on the tip of the valve. |
| 03:21 | This is essential to allow a reasonable cam profile to be used. |
| 03:27 | Secondly, the bucket will be responsible for adjusting or setting the valve clearance. |
| 03:33 | This clearance is the distance between the surface of the bucket and the cam lobe when the valve is in the closed position, and it's often referred to as a valve lash. |
| 03:44 | Valve lash can be set mechanically, either by special shims of varying thickness that are placed between the bucket and the valve tip, or by actual buckets or by actual buckets that are manufactured in varying thicknesses and selected to achieve this required clearance. |
| 04:02 | Due to the maintenance required to check and adjust valve lash as the engines age, it's more common in modern engines to find hydraulic lash adjusters, which use oil pressure to maintain and automatically adjust valve lash. |
| 04:18 | A fundamental difference here is that a hydraulic lash adjuster will take up all of the clearance between the bucket and the cam, while a mechanical system will always have a small amount of clearance. |
| 04:31 | Another common valve actuation technique with overhead cam cylinder heads is where the cam lobe operates against a rocker arm or finger follower which then actuates the valves. |
| 04:43 | A rocker arm is simply a rocker that is located on a shaft in the cylinder head, where one end runs against the cam lobe, and the other against the tip of the valve. |
| 04:54 | Often there will be a mechanical lash adjuster on the rocker arm, where it contacts the valve tip so that the valve lash can be adjusted. |
| 05:04 | A finger follower is similar in design to the rocker arm, however, rather than using a rocker shaft, one end of the finger follower locates on a pivot in the cylinder head, and the other locates on the tip of the valve. |
| 05:18 | With a finger follower design, the pivot point is used to adjust valve lash, and the pivot can consist either of a mechanical or hydraulic adjuster. |
| 05:29 | A subtle aspect of rocker arm or finger follower valve actuation is that the lift provided by the cam shaft is effectively multiplied by the rocker ratio. |
| 05:40 | What I mean by this is that if the cam lobe provides 5 millimetres of lift and there's a two to one rocker ratio, then the valve would actually be lifted by 10 millimetres. |
| 05:52 | The rocker ratio is the ratio of distance between the pivot point and valve tip, to the pivot point and cam lobe. |
