Once mechanical losses have been minimized, the primary way BHP is increased is to improve the volumetric efficiency. An improvement in VE implies an increase in fuel consumption to produce the additional power. It does not necessarily reduce thermal efficiency (BSC). In some cases this can be improved. The volumetric efficiency is the ratio between the volume swept by the piston and the volume of fuel vapour pumped into the cylinder in one stroke. That is the percentage filling. VE is usually fairly high in full size engines around 70-80% at modest RPM. Modern four stroke engines are said to achieve figures as high as 90%. This may also be referred to as the scavenge efficiency. The efficiencies quoted are based upon the intake volume relative to swept volume; this is satisfactory for four strokes because both Induction and exhaust strokes are similar. With two strokes using the crankcase as a pump the displacements and compression ratio of pump and cylinder are not closely matched by default. Hence volumetric efficiency in two strokes engines is modified by the applied pump delivery ratio. With small model engines it is difficult to equal the figures quoted, as the pumping ratios cannot be matched due to the comparatively large crankcase volume. The VE however is not totally dependant on the PDR as the engine can only take in the volume (mass) which has exited to exhaust during the scavenge process
Typical methods of improving volumetric efficiency are –
- Super charging (pressure fuel/mixture feed).
- Increasing the pumping ratio.
- Enlarging ports.
- Modifying timing.
- Port flow direction control.
- Induction tract design modifications.
- Exhaust extensions.
SUPER CHARGING
The induction system is pressurized above atmospheric conditions by means of a compressor. Two systems are used to apply the drive, most common is that where exhaust gas is used to drive a turbine coupled to the blower. Known as turbo charging, this system provides high thermal efficiency and high power at modest RPM. For higher pressures the compressor is mechanically driven from the crankshaft as in the case of Dragsters. For two strokes an alternative to this is what is known as a pump motor. Pre 2nd world war DKW racing motor cycle engines used this system. An extra piston is connected to the crankshaft moving in phase with the main cylinder to increase the crankcase displacement. Pete Buskell produced a prototype like this in the 60's. A further system in current use by model applications is pressure fuel feed. This permits a larger choke to be applied, secondly, dependant on the jet location, the crankcase pump is pressurized by the incoming fuel. In other words this is no longer limited by atmospheric pressure as flow is determined by pressure at the jet. This provides a significant power advantage with fuels such as methanol, and nitro methane (these require a rich mixture). A consequence of this is that test reports should state what fuel feed system has been applied during performance evaluation. Some model engine classes restrict the applied system.
PUMPING RATIO
Transfer (Scavenge) occurs due to stored energy created during the pump compression stroke.
The most important factor is the size of the Delivery Pump in relation to the Volume to be scavenged. This is obvious if you consider a Supercharged Uniflow Design or assume no loss of charge.
The Volume to be scavenged, is the Swept Volume plus the Clearance Volume. The Swept Volume in this case is the engine capacity; that is the volume generated by the full stroke. It must not be confused with the Trapped Volume. Some books may specify a pumping ratio based upon the Trapped Volume, this is not correct because this does not include the additional volume added when the Exhaust port is open. With a Naturally Aspirated Two-Strokes the size of the pump is very restricted. It is not feasible to equal or exceed this Volume without a secondary pump. The pumping stroke is that from Induction Port Close to Transfer Port Open, hence the Delivery Pump Displacement is less than the Volume to be scavenged. Due to this, Efficiencies in excess of 50% are difficult to achieve.
A Naturally Aspirated Two Stroke is unlikely to have a Geometric Compression Ratio greater than 1.6 to 1. The actual ratio achieved is somewhat less because the pumping stroke is from Induction Port Close to Transfer Port Open producing about 1.5 to 1. No further compression occurs following Transfer Open because both Pump and Combustion chambers are connected and the Exhaust port is open. The outcome of this is that both chambers drop pressure simultaneously as Exhaust continues. Only a displacing action takes place between the two chambers as a result of piston movement (the pressure is equal on both sides of the piston). There is a substantial difference in Gas Temperature between the two chambers at the start although the pressure is equal. The displacing action cools the cylinder contents as the Transfer Charge replaces and mixes with burnt gas. The Temperature in both chambers drops due to Expansion in the process ready for the next cycle.
Generally the crankcase volume should be kept to a minimum, providing the highest pressure differential to atmosphere. However there are conditions at high RPM where flow through the various ports and passages has to take precedence. In other words a compromise is made to suit the characteristics required.
PORT SIZE
The Rate of Flow is restricted by the size of Exhaust, and Transfer ports. Orifice coefficients are applied to determine discharge rates. There is an optimum size for all ports. Actual size is usually restricted by the engine's physical dimensions. The art is to achieve the ideal combination to suit the desired performance. Large cylinder ports can be achieved by timing changes, but the results may not be what you would expect. This method is used when a tuned pipe is fitted. Without the pipe more conservative timing usually gives higher performance.
TIMING
During crankshaft rotation ports are opened and closed in sequence to enable a cycle of operations to take place. See The Timing Diagrams.
PORT FLOW DIRECTION CONTROL
When flat top pistons are employed, flow direction is crucial to both scavenge and thermal efficiency. The loop flow port cylinder/piston configuration is a classic example of this. The bypass entry angles into the cylinder are used to control the gas flow. In earlier designs deflectors were fitted to the piston. Needless to say both methods work well. However attention must be given to all entry and exit points otherwise results can be disappointing.
There are three principal systems employed to control direction (see Flow Diagrams). The principal debate regarding alternative Scavenge Systems is mainly associated with the degree of mixing, which occurs, and the prevention of fresh charge loss out to Exhaust. A 4th system known as Reverse-Flow is in certain model engine designs. In this case the cylinder inlet ports are directly below the exhaust, most feature radial ports.
The relative Scavenge Efficiency of alternative porting layouts is difficult to establish, the following curves are based on empirical, out of date, data. It is virtually impossible to obtain 100% efficiency even if supercharging is applied, there will always be some mixing or residual gas as part of the cylinder contents. The significant difference between the Crossflow and Loopflow curves can most probably be attributed to Exhaust Pipe design. Modern designs raise the cylinder pressure above atmospheric pressure at the point of exhaust closure increasing the quantity of mixture available for combustion. This is a form of supercharging, the quality of this mixture is not by implication better due to this, and the reflection pulse charge contains a mixture of exhaust gas from the previous cycle. With pure supercharging and a Uniflow system the quality of mixture is superior because there is normally sufficient pressure to prevent back flow, however this can lead to high fuel consumption. Users should appreciate that ratios for Scavenge Efficiency cannot be applied from the this data for use by the program. The graph has been included here to clarify the debate related to this subject and the alternative porting layout.
Engine Scavenge Efficiency (Volumetric Efficiency) is determined by two factors, these are its pumping capability and Porting layout. The rates of flow determine the primary efficiency, this is limited by the pumping ratio. Because the Exhaust port is open during the process it is possible for the Scavenge charge to short circuit to Exhaust, correction is required to account for this. An Option is provided to set potential values for a range of Scavenge layouts. It must be appreciated that the application of empirical data for this purpose can be misleading because it only reflects the state of development at that time. A further factor is that the various Scavenge diagrams only indicate the projected, or desired flow. The probability is that the extent of short-circuiting, which occurs, has a greater dependency on the engines timing and exhaust system.
The values set are used in the calculation process, however the actual values output are modified by the Scavenge flow. In effect they only specify the maximum which may be achieved by a particular layout. Hence these values should be considered as the short efficiency of the specified layout.
Individual port efficiencies, which affect the Scavenge Efficiency, are set in the engine database.
- See Engine Data
Exhaust, Transfer, Boost, and Induction Coefficients.
INDUCTION TRACT DESIGN MODIFICATIONS
Theoretical tuned lengths of Induction tracts are based upon the Speed of Sound; for model size engines the required lengths exceed that currently applied. Calculations for my Dynamic 21D indicated a requirement of 8 inches at 15000 RPM. The principal reason for this is attributed to boundary layer effects. The possibility exists that the pulse theory for two stroke induction systems is not correct because the pulse direction can be reversed by scavenge conditions in the crankcase pump, hence gas inertia in the induction tract is of greater importance as this pressurizes the crankcase pump beyond TDC up to port closure. There is a total lack of empirical data around this subject it could be a useful project to undertake. Test work is much easier with rear induction motors than front rotary designs. It should be noted that the Induction passage through the crankshaft is part of the tuned length; hence the length of such types already exceeds other concepts. This may be one of the possible reasons for the apparent superiority of current front rotary designs, rear disc induction lengths being too short.
EXHAUST EXTENSIONS
This is an important aspect which few individuals have explored. Kevin Lindsay did early experimental work on this subject. There is definite resistance to such devices by free flight modelers.
Copyright ©2008 Gordon Cornell