- One of the key features of MOTA 5 is that it can run
engine simulations. It will display the engine
graphically. It will allow a set of performance curves to
be plotted as graphs with engine power and torque at an
engine speed. The dynamic wave display allows a visual of
the propagation of pressure, velocity, temperature,
density and the purity of waves in each duct. This
includes the exhaust pipe. Additional selections allow
simulations of the reed or rotary valve at speed. Output
data shows the cylinder, exhaust pressure difference and
the scavenging ratio, complete with delivery ratios,
power and torque, BMEP, PMEP, FMEP, IMEP, brake specific
fuel consumption, flow ratios - the delivery and exhaust
gas ratios, the mass based scavenge ratio, the volume
based scavenge ratio, scavenging efficiency, the
percentage of energy lost due to heat through the exhaust
and the peak cylinder temperature and pressure.
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- The MOTA 5 programs allow the design of engines
parameters when the input is entered in metric units of
measurement. The selections from the menu and submenu's
are:
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- Create or Edit an Engine Data File
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- Run MOTA - The Engine Simulator
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- Display Output Graphics
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- View an Engine Specification and Performance
File
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- Performance Curves Calculator
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- Expansion Chamber Cone Construction
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- Within the software is a section called dimensioning
utilities. It is very helpful when trying to determine
some of the more unusual engine parameters. For instance
if you know the piston pin is offset 1 mm in the positive
direction (toward the direction of engine rotation) the
program will calculate where BDC is - in this case 180.29
degrees ATDC. Offset the piston pin another mm and BDC
moves to 180.58 degrees ATDC. Isn't it interesting to
know that small changes in the offset of the piston pin
can have a noticeable effect upon where these positions
end up.
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- There are some terms which may need clarifying, for
example - the volume of the combustion chamber when the
piston is at top dead center is referred to as Cylinder
Clearance Volume. The Crankcase Clearance Volume is
defined as being the volume below the piston crown when
the piston is at BDC. MOTA will allow the design of
engines that are either piston port, rotary valve or have
reed valves. For reed valve engines there is input for
reed petal thickness and type of material as well as
whether the thickness is constant over the total length
of the petal, the width, unclamped pedal width and
density of the petal material. There is also input
pertaining to the values of Young's Modulus and for reed
petals that overlap one another (Boyesen). The reed valve
block is broken down into several more specific
dimensions for precise data entry. This includes the
number of ports, block angle, width of each port, corner
radii, length of port and the stop plate radius. The term
"tip deflection" refers to the amount of distance a reed
petal can open. For disc valve equipped engines there is
input for the opening, closing, height, width, corner
radius and radius to the bottom of the port. MOTA uses
what is called Fuel Calorific Value. This relates to the
octane of the gasoline used in the design. This value is
set as a default to 98 octane leaded gasoline. There is
input for different air/fuel ratios at different engine
operating speeds.
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- Transfer port input is classified as the number and
type of duct. There is input for the transfer duct
dimensions. There can be as many as 3 pair of transfer
ports and 1 additional (boost) port. There is input data
describing the sectional area of these passages. Engine
port dimensions are entered after measuring them using
one of the methods available. This can be as simple as
knowing the port height, width and top and bottom corner
radius. Additional port dimensions can be entered using
what is called profiled data. This is defined as entering
a port shape with as many as ten different intervals
(sections) associated with the port width. Here you can
enter bridged ports and auxiliary exhaust ports (finger
ports). The term Axial Attitude Angle refers to the port
roof angle. The term Radial Attitude Angle refers to the
way vertical port walls are recorded. It is different
than the way other software programs determine it. The
scavenging cycle is given particular attention and is
described as having one of four different
characteristics. Inlet duct dimensions may have data
entered in as few as 2 and as many as 4 sections. These
sections are described as being the inlet pipe sections.
This includes any section after the carburetor bell mouth
and may have its parameters entered as length, diameter
of inlet and diameter of outlet.
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- MOTA has an input category to simulate the engine
whether it is equipped with an exhaust control valve or
not. Additionally, it asks for the number of sections the
tuned exhaust pipe has which is fitted to it. For
simulation, the exhaust pipe data entry is input in as
many sections as necessary to accurately describe the
pipe - this includes section length and width. When I
entered my data I subtracted the thickness of the pipe
material to keep the dimensions tight. As a separate
design parameter there are input for 2 different types of
mufflers, classified as integrated or separate.
Integrated mufflers are those that more closely comply to
pipes with internal stingers. I choose a separate muffler
for my engine. Ignition and combustion efficiency are
represented and described in degrees of crankshaft
rotation. There is input to change the ignition timing
(advance or retard) of an engine at different operating
speeds.
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- The MOTA 5 simulator is unlike the Dynomation 2
simulator in that it does not display a graph as the data
is run through the simulation process. Its information
appears on the screen as numbers within columns. This
method of display, though not as visually appealing is
less confusing because there is much less activity on the
screen. The hard data is displayed without any bells or
whistles. My initial simulation run(s) indicate a
decrease of power during the 40 run set - I suspect this
is due to heat. The power (13 hp) at 5000 was slightly
higher than it was at 4000 RPM as is the mean exhaust
temperature - it's simulated at 1092 degrees. After the
first 2 sets of runs (4000 RPM & 5000 RPM) the screen
displayed a couple of graphs which indicate the
power/torque and peak cylinder temperature and pressure.
At 6000 RPM power is up to 18 hp and the exhaust
temperature is down about 100 degrees. At 7000 RPM again
power is up. This time it shows 24 HP with an exhaust
temp of 1038 degrees. At 8000 its reading 24.6 HP and the
exhaust is 1047. At 9000 power falls off to 21 HP with
the exhaust temperature of 1050. It made me want to try a
leaner fuel to air ratio - I think power would be
increased. Currently the fuel mixture is set at 12:1. I
altered the fuel mixture and ignition timing in the
examples below to indicate the changes MOTA 5 can do. The
values are listed like this ... RPM = HP (Exhaust gas
temperature). Peak power is indicated in red.
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- Update
9/18/99 - As usual an
enthusiast pointed out a serious error I have made while
doing this testing. This one comes from a man named Steve
who reminded me that changing the fuel mixture in the
direction I indicate below is making the mixture RICHER
not leaner. So it seems the richer I made the mixture the
more power the simulated engine made. I'll have to re-run
the simulator in the leaner direction (this time for
sure) and see what my outcome is. Thanks Steve
:)
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- Baseline settings - 12:1 fuel mixture, ignition
timing set at 10 degrees BTDC - 5000 = 13 (1092), 6000 =
18 (997), 7000 = 24 (999), 8000 =
24.6 (1038), 9000 = 21 (1050).
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- Lean the mixture from 12:1 to 11.8:1 - 5000 = 15
(937), 6000= 19.6 (947), 7000 =
26.5 (950), 8000 = 26.44 (980), 9000 = 21.1
(958)
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- Changed fuel mixture to 11.4:1 - 5000 = 18.59 (1005),
6000 = 20.3 (969) 7000 = 27.25 (982),
8000 = 28.22 (1014), 9000 =
25.25 (1001)
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- Changed fuel mixture to 11:1 - 5000 = 19.6 (1082),
6000 = 20.82 (992) , 7000 = 28.09 (1005),
8000 = 29.48 (1044), 9000 =
25.8 (1035)
Changing the combustion burn period to 50 degrees of
crankshaft rotation (from 55 degrees). Fuel mixture 11:1.
5000 = 20.05 (1084), 6000 = 21.3 (1008), 7000 = 28.77 (991),
8000 = 30.10 (1026), 9000 =
25.8 (1017)
Changed the combustion efficiency to .82 from .85. Fuel
mixture 11:1 - 5000 = 19.53 (1075) - peak temps very high
(1260), 6000 = 20.9 (951) canceled.
Changed ignition timing to 15 BTDC. Fuel mixture 11:1.
Burn period changed back to 55 degrees - 5000 = 20.6 (1078),
6000 = 22.1 (958), 7000 = 29.71 (967),
8000 = 30.28 (986), 9000 = 25.2
(945)
Changed timing to 17.5 BTDC and 11:1 fuel mixture. Burn
period 55 degrees - 5000 = 20.94 (1074), 6000 = 22.5(941),
7000 = 30.3 (943), 8000 = 30.4
(971), 9000 = 24.86 (950)
Timing set back to 12 degrees BTDC. Fuel mixture 11:1 -
5000=19.37(1115), 6000= 19.9(987), 7000= 26.2(992),
8000= 29.94 (1041), 9000= 26.71
(970)
It will be interesting to see how this engine does on a
real dynamometer. It's hard not to notice that pressing a
few keys to change the ignition timing and jetting is much
faster and simpler than actually changing the parts on the
real engine. As I changed the timing and other variables, I
was able to increase the power output. The most noticeable
change in power is before and after the power peak.
More to come - Rick
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