Design and Simulation of Two Stroke Engines

Chapter 6 - Empirical Assistance for the Designer
higher and a lower level is shown as the most informed view of a more complex relationship
coming from the "logic" above.
The arithmetic relationships from those diagrams are rewritten below. In each equation
the unit of bmep is bar and the specific time area is in units of s/m.
For inlet ports, the specific time area is labeled as Asv{.
bmep = 774 Asvi -1.528 (6.1.9)
For transfer ports, the upper band of specific time area is labeled as Asvti-
bmep = 2400Asvti - 9.66 (6.1.10)
For transfer ports, the lower band of specific time area is labeled as Asvt2.
bmep = 587Asvt2 +0.128 (6.1.11)
For exhaust ports, the specific time area is labeled as Asve.
bmep = 1050Asve - 5.975 (6.1.12)
For exhaust blowdown, the specific time area is labeled as Asvb.
bmep = 8187Asvb + 1.75 (6.1.13)
However, while these functions are easily solved using a pocket calculator, a simple computer
program is referred to in the Appendix Listing of Computer Programs as Prog.6.1,
TTMEAREA TARGETS, and is available from SAE. For those not very familiar with programming
in Basic this straightforward computer program will serve as another useful example
of data insertion, simple calculation, and data presentation on both the computer screen
and the printer.
As an example of the use of this program and of the analysis represented by Eqs. 6.1.7-
6.1.11, consider the case of two engines similar to those studied in Chapter 5, a chainsaw
engine and a racing engine. As observed there, these engines are at opposite ends of the
performance spectrum. Imagine that a design is to be formulated for two new engines to
satisfy the very criteria which are known to be attainable by virtue of both the measurements
and the engine modeling carried out in Chapter 5 on such engines. The design brief might
read as follows:
(a) A chainsaw engine is needed to produce 3.9 kW at 9600 rpm, with a bmep of 3.75 bar
as a potentially obtainable target. That this would necessitate an engine of 65 cm 3
swept volume is found from Eq. 1.6.6 in Sec. 1.6.1. All of the porting is to be piston
controlled.
(b) A high-performance 125 cm 3 engine is needed for racing, where 26.5 kW is required
to be competitive. For mechanical reasons, it is decided to try to produce this power
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