 |  | Engines for Biogas (GTZ, 1988, 133 p.) | ||
 |  | 6. The Gas Otto Engine | ||
|  | 6.1 Necessary Modification | ||
| | 6.2 Performance and Operational Parameters | ||
| | 6.3 Design of Mixing Devices | ||
| | 6.4 Change of Compression Ratio | ||
| | 6.5 Manufacture and Installation | ||
| | 6.6 Control | ||
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The modification of an Otto engine (spark ignition, petrol or gasoline engine) is comparatively easy as the engine is designed to operate on an air/fuel mixture with spark ignition. The basic modification is the provision of a gas-air mixer instead of the carburetor. The engine control is performed by the variation of the mixture supply, i.e. the throttle valve position as has been the case with petrol fuel.
An increase in the compression ratio appears to be desirable as it provides an increase of the efficiency of the process from the mere thermodynamic point of view. A lower specific fuel consumption and a higher power output can be expected. The modification is however permanent and prevents operation on original fuel in cases of biogas shortage.
The adjustment of the point of ignition in relation to the slow burning velocity of biogas imposes no specific problem as a standard ignition system provides for adjustments in a sufficiently wide range.
Engines which cannot operate on unleaded fuel will miss the lubrication effect of condensing lead especially on their exhaust valves. They are therefore subjected to increased wear and tear in gas operation.
Gas Otto engines when modified from Otto engines using petrol are found to produce less power than in the petrol version. The reason is the decrease in volumetric efficiency as a gaseous fuel occupies a larger portion of the mixture's volume sucked into the engine than liquid fuel and displaces air accordingly. The liquid fuel has a higher volumetric energy content than gas and also cools the air/fuel mixture when evaporating in the intake manifold. The cooling effects an increase in density, and hence the amount of air/fuel mixture actually sucked into the engine on a mass basis is higher.
A gas engine, especially when operating on biogas with a large proportion of useless carbon dioxide, can suck a reduced amount of air only to allow room for th necessary amount of fuel gas. As in Otto engines an excess air ratio of l = 1 ± 0.1 has to be maintained and the inlet ducts and manifolds are dimensioned for operation with petrol, the total fuel energy in a mixture of biogas and air is less than in petrol operation. With the decrease in the maximum possible supply of fuel energy or the energy density of the mixture (mixture heating value) the maximum power output consequently decreases in the same proportion. The rate of decrease in power is largely dependent on the volumetric heating value of the gas, e.g. biogas with 70% CH4 has a higher volumetric calorific value than biogas with 50% CH4 only. The power output of an engine is therefore higher in operation on gases with high calorific value than in operation on "weak" gases. Biogas (60% CH4) with a calorific value of Hu = 25 000 kJ/nm³ ranges as a medium weak gas and causes power reductions of about 20% (purified methane or natural gas 10%, LPG 5%). The main effect of the reduction of power is that it needs to be well considered when selecting the power class of an appropriate engine for a given application with a specified power demand (see Chapter 7).
The engine's power and speed control is performed by a variation of the supply of the air/fuel mixture to the engine. This is achieved by the operation of a butterfly valve situated between the actual mixing device and the engine inlet. Closing the butterfly valve effects a pressure drop (throttling effect) in the flow of the mixture by which the cylinder is filled with a mixture at lower pressure ps, hence with a lower amount of air/fuel mixture on a mass and energy basis. As a result the power output, the mean effective pressure and the efficiency decrease in controlled (partial load) operation. The effect of the decrease in efficiency is realized in an increase of the specific fuel consumption in partial load operation (see Fig. 6.1). To compensate for the above-mentioned effects the engine should rather be operated at medium speeds but with open throttle. This requires an appropriate combination with the speed and power requirements of the driven machine as explained further in Chapter 7.
Fig. 6.1: Performance diagram of an
Otto engine using liquid fuel (——) and methane(—·—)
alternatively (Rodagas).
1 power, 2 torque, 3 specific fuel consumption.
The mixing device has to ensure the provision of a constant air/fuel ratio irrespective of the actual amount sucked into the engine, i.e. irrespective of the butterfly valve position. This is achieved by adequate design of the mixing device, whether a venturi mixer or a suction-pressure controlled mixing valve. A simple mixing chamber however requires a control of the fuel gas flow together with the main butterfly valve, i.e. it cannot provide a constant air/fuel ratio by its design alone.
Before a specific mixing device is chosen, the necessity/possibility of another type of fuel for cases of insufficient biogas supply should be considered. The different fuels and their technical requirements are given below:
- LPG, natural gas: mixing valve or venturi, with pressure reduction valve (50 mbar) before gas inlet. Maximum compression ratio e = 11. Simple mixing device for biogas can be used with adjustment at gas inlet (for operation at constant conditions).
- Alcohol: carburetor, similar to petrol version but with main jet enlarged in the ratio of calorific values of petrol/alcohol. Petrol carburetor can be retained. Maximum compression ratio e = 12.
- Petrol: previous petrol carburetor retained or remounted. Maximum compression ratio e = 9.5 for premium, e = 7.5 for regular.
6.3.1 Venturi Mixer
A venturi mixer utilizes the same fluid-mechanic effect as a standard carburetor, i.e. the change in airflow quantity and velocity causes a change in pressure at the channel contraction which in turn effects a change in flow of another medium (fuel) to join and mix with the main airflow in the required proportion.
Fig. 6.2: Venturi mixer with gas
supply through several bores.
c1 velocity at mixer inlet,
cv velocity at venturi contraction, di diameter of
mixer/engine inlet, dv diameter of venturi contraction, ci
velocity of mixture at engine inlet.
Fig. 6.3: Venturi mixer with a single
gas inlet nozzle.
dg diameter of gas inlet nozzle, other symbols
as in Fig. 6.2
The venturi principle functions as follows:
For high air volume flow:
- Air velocity is high.
- Air pressure is low at the
contracted cross-section.
- The pressure difference between fuel gas and
airstream is high.
- Much fuel gas flows through the openings to mix with the
airstream.
For low air volume flow:
- Air velocity is low.
- Air pressure is high at the
contracted cross-section.
- The pressure difference between fuel gas and
airstream is low.
- Little fuel gas flows through the openings to join the
airstream
The following procedure shall give a general representation of the dimensioning of a(self-made) venturi mixer.
Step 1:
Determine the volumetric intake V, (in m³/s) of the engine as a function of engine cubic capacity V1 (in m3/s) at rated or maximum operational engine speed n (in 1/min or rpm), see Equ. 3.17 and 5.4:
Step 2:
Determine the mean intake velocity c; (in m/s) of the venturi mixer using the channel's cross-sectional area Ai (in m²), see Equ. 5.1 and 5 5 :
whereby 
The cross-sectional dimension of the venturi mixer should be equal to that of the manifold. The intake velocity ci almost equals the velocity ci of the air coming from the air filter when the throttle is fully opened. In a controlled position the velocity before the butterfly valve (in flow direction) is reduced. The dimensioning of the inlets for fuel gas, however, needs to consider the fuel requirement at unthrottled operation for maximum performance, i.e. at maximum intake.
Step 3:
Determine the cross-section of contraction. The contraction in the venturi mixer will cause the airflow velocity to rise as a linear function of the change in the cross-sectional area. The velocity at the contraction or "bottleneck" of the venturi cv should not exceed cv = 150 m/s at maximum flow rate. The "bottleneck" or venturi area Av is found by
(Equ. 6.1)
Its diameter dv is found accordingly:
(Equ. 6.2)
The shape of the contraction has an influence on the flow in a
sense that the more abrupt the change in area is, the more extra friction and
separation of the flow from the channel wall occur.
The venturi shall
therefore be evenly shaped following the example given in Fig. 6.2. The aperture
angle on the downstream side shall not exceed 10°. The contraction side
upstream is not so sensitive and is often shaped in a roundish profile as can be
seen in Fig. 6.2. Standard carburetors use similar venturi profiles.
Step 4:
Determine the required biogas fuel flow. The main parameters for the determination of the fuel flow are the
- engine operational power,
- calorific value of the biogas
as per volume (Hu,vol),
- specific fuel consumption of the engine
or the efficiency respectively.
The specific fuel consumption of the engine or the efficiency are not always known especially in second-hand or reconditioned engines. However, as a rough figure for Otto engines h = 0.25 or sfc = 4 kWh fuel/kWh mech. energy can be chosen. The fuel and the gas volume flow required can be calculated in accordance with the procedure used in Chapter 5.2.1.3 for diesel engine mixing chambers. In an Otto engine, however, the fuel gas provides 100% of the required fuel as no other fuel (dual fuel mode) is supplied, i.e. Step 2 in the above-mentioned procedure is not required for Otto engines.
Step 5:
Determine the area of the fuel gas inlet, Ag. The fuel gas inlet at the bottleneck of the venturi jet can have different shapes (see Figs. 6.2 and 6.3):
- several openings around the circumference of the venturi jet
being fed by a ring channel, or
- pipe with-one opening.
When the second alternative is chosen the area occupied by the fuel gas pipe Ag in the core of the venturi has to be subtracted when establishing the bottleneck cross-sectional area of the venturi.
The effective area 
is
therefore
(Equ. 6.3)
whereby 
The flow velocity cv in the anular clearance at the
area shall also not exceed 150 m/s. The
cross-sectional area of the gas inlet Ag is then established similar
to the procedure in Chapter 5.2.1.3.
whereby the flow velocity of the fuel gas in the jet/nozzle is
The active pressure difference Dp for the fuel gas flow is established between the pressure in the gas supply pipe before the mixer (i.e. the biogas plant pressure minus the pressure losses caused by the flow resistance in the gas piping system up to the connection at the mixer) and the pressure in the venturi bottleneck where the gas flow joins the airflow.
Fig. 6.4: Biogas flow through a
nozzle cg as a function of the active pressure difference /p.
1 gas density
p=1.2 kg/m³.
The pressure in the gas supply pipe ranges at 0.005 ... 0.02 bar
gauge. The pressure in the venturi bottleneck is a function of the contraction
of the venturi, the actual airflow rate and the pressure reduction caused by the
air filter. It can be calculated
using Bernouilli's equation (see Equ. 5.2):
so that the pressure at the venturi bottleneck pv is
(Equ. 6.4)
The velocity at the venturi bottleneck is found using the continuous flow equation (see Equ. 5.1) (for incompressible media)²:
with the previously calculated parameters (see Equ. 6.2) for the intake velocity ci at fully opened throttle
The volumetric intake of the engine Vi can also be used to determine the velocity cv:
when the venturi bottleneck area Av is already known. As mentioned, the venturi bottleneck area is to be established in such a way that at maximum volume flow rate Vi the velocity at the bottleneck ranges between cv = 100 ... 150 m/s. A smaller bottleneck diameter increases the venturi velocity while a larger one decreases it respectively.
As s rule; of thumb and for first calculations the diameter ratio for a venturi may range at dv/di = 0.67 which would result in a velocity ratio of cv/ci = 2.25, e.g. a velocity increase from ci = 50 m/s to cv = 112.5 m/s. Fig. 6.5 gives the relation between the diameter ratio and the velocity increase to some selected velocities at the venturi bottleneck.
A scientifically precise calculation of the fuel gas inlet area would require a precise determination of the pressure of the gas at the calibration valve of the venturi, the fuel gas temperature and its composition as well as a high precision manufacturing standard. However, in biogas applications the volumetric calorific value often differs with plant performance and ambient parameters. Furthermore building a venturi mixer should consider its applicability for more than only one specific engine operating at one specific biogas plant.
Due to these non-uniform boundary conditions the layout of the venturi shall be based on assumption of "unfavorable" conditions for the calculations of the calorific value and the pressure drop in the biogas system up to the venturi mixer. If this results in slight overdimensioning of the fuel gas inlet area Ag (whether single nozzle or several bores), the calibration valve can be partly closed, imposing an additional but controllable resistance in the fuel gas supply system. The venturi gas mixer can thus always be adjusted to the actual fuel gas conditions. The additional advantage is that it provides a possibility for manufacturing venturi mixers in small series for similarly sized engines and different gases if required by the market.
A similar approach is used by the commercial manufacturers of pressure-controlled gas mixing valves and venturi mixers, i.e. "Impco" and "Rodagas". All mixing valves and venturis are equipped with a fuel gas calibration valve for mixture adjustment.
The calibration of venturi mixers and gas mixing valves is done during operation at the maximum required power and speed. The gas calibration valve is at first kept fully open and the engine warmed up. It is then gradually closed until the engine begins to lose power/speed, and carefully opened again until the required set point is reached again. The calibration valve should be fixed in this position. An additional control of the CO content in the exhaust gas is recommended; the CO value is optimal at 1.0 + 0.5% Vol.
Fig. 6.5: Venturi diameter ratio
dv/di as a function of intake velocity ci and
the required velocity at the venturi bottleneck cv
1 cv
= 100 m/s, 2 cv = 120 m/s, 3 cv = 150 m/s
Idling, if necessary, can be adjusted with the lever operating the butterfly valve in such a way that a small clearance is left for the idling amount. Some mixing valves have separate idling screws.
6.3.2 Pressure-Controlled Mixing Valves
Pressure-controlled gas mixing valves are in frequent use for motor vehicles which are driven by LPG. They are manufactured in large series and in different types and sizes for differently sized engines. As the manufacturing of these valves uses rather sophisticated methods and materials not everywhere available, it does not appear recommendable to try self-manufacture. The selfmanufacture of a venturi involves far less effort in terms. of material equipment and skills while it provides a technically sound solution as well.
Fig. 6.6: Cross-sectional view of
gas mixing valve.
1 butterfly throttle valve, 2 diaphragm, 3 spring, 4 gas
valve cone, 5 mixture adjustment valve, 6 air bypass adjustment, 7 air inlet, 8
engine inlet, 9 bore for suction pressure, connects M and R, A space of air
inlet before mixing zone, M space of mixture flowing to engine inlet, R space
behind diaphragm, connected to M via bore (9).
Fig. 6.7: Gas mixing valve in
operation, schematic (Impco). S metering spring, D diaphragm, P vacuum transfer
passage, V gas metering valve, I idle air bypass adjustment, A power mixture
adjustment, T throttle valve.
The operation of the engine (Fig. 6.6) produces a suction pressure ("vacuum") in space M which is passed on to the space R behind the diaphragm [2] via a bore [9]. The space A is connected to the air intake and has almost ambient pressure conditions. The pressure difference between A and R forces the diaphragm to move against the force of the spring [3]. The valve now allows air to pass from A into M through a gradually opened, calibrated ring channel.
Simultaneously the fuel gas can now pass through an opening controlled by the valve cone [4]. The air and fuel mixed in space M are sucked into the engine intake [8] via the butterfly throttle [1]. (See Fig. 6.7 for demonstration of mixing valve in opened position.) The more the throttle is opened for more power, the more the vacuum from the engine intake becomes effective in the spaces M and R, and hence the more air and gas are allowed in through their increased openings. The air/ fuel ratio remains constant as required because both the ring channel and the valve cone have been shaped accordingly. Variations in gas quality (pressure, calorific value) can to a certain extent be compensated by the mixture adjustment or calibration valve [5] which acts as a throttle in the gas supply changing the active gas pressure at the opening, hence the amount of fuel mass entering (in other words, the calorific value of the fuel on a volume basis). A modification of the internal structure of the mixing valve is not practicable and should be avoided.
In places where these LPG mixing valves are easily available they may be used as long as the calorific value of the biogas is not lower than about 25 000 kJ/m³. The gas inlet opening inside the valve has been dimensioned for LPG with a much higher volumetric calorific value than average biogas. The gas inlet will therefore be too small for weaker gases and may produce an air/fuel mixture too lean for good performance.
6.3.3 Introduction of a Constant Pressure Control Valve
A constant pressure valve helps to provide a constant pressure in the biogas supply pipe from the biogas plant. Whenever the biogas pressure is likely to fluctuate in a range of more than 20 mbar or to become higher than 50 mbar before the mixing valve or venturi, a constant pressure (pressure reduction) valve should be introduced and mounted into the biogas pipe before the mixer. Higher fluctuations in biogas pressure would result in corresponding fluctuations of the volumetric calorific value and unbalance the setting or calibration of the mixer, hence the performance of the engine.
Constant pressure valves are always necessary when the biogas is supplied by means of a blower or when LPG is used as an auxiliary fuel in case of biogas shortage. Whenever it can be foreseen that the gas pressure will continuously be rather low (i.e. Iower than 5 mbar) a pressure reduction valve should not be introduced as it produces a small but disadvantageous extra pressure drop even if it is fully open.
Pressure reduction valves are commercially produced in many
varieties and specified by their pressure, volume flow rate and type of gas. For
more information refer to manufacturers' overview
in Chapter 10.
6.3.4 Simple Mixing Chamber
A simple mixing chamber or even T-joint tube-type mixer may provide an alternative for one special application. This is the case when the engine is operated steadily at one load and one speed, i.e. when the driven machine guarantees a steady power demand. Equally important is the respective calibration of both air and fuel gas supply.
The mixing chamber can be designed in accordance with the criteria stipulated in Chapter 5.2. The control of the power or the point required by the operation is done with one valve each in the air and gas supply and requires experience in finding the required air/fuel ratio. So-called "feeling" is rarely reliable enough to assure operation at the required air/fuel ratio. Another possibility is the provision of a butterfly valve for the mixture, a hand-operated valve for the fuel gas and an uncontrolled air inlet from the air filter, in other words a mixer similar to the venturi type but without the venturi nozzle ring.
It should be borne in mind that even if the simple mixer is properly calibrated or set at one specific point of operation, a change in power demand from the driven machine will change the speed of the engine, hence the volume intake, and cause a disproportion in the air/fuel ratio, unlike in a venturi or gas mixing valve. Small variations may be acceptable as long as the driven machine tolerates speed fluctuations. In case of larger power demand fluctuations the control has to be readjusted in due course by operating personnel as the engine can be damaged by running on an improper mixture or at overspeed.
Only few applications may allow the use of simple mixing devices under the mentioned limitations. These are
- an electric generator with a reliably controlled constant power output and a network with a corresponding demand, and- a centrifugal pump delivering a constant flow rate of water against a constant head.
Standard petrol engines operate at compression ratios of e = 7 ...9 so that self-ignition of an air/fuel mixture is impossible. The efficiency and power output can in principle be improved by an increase of the compression ratio to E = 11 . . . 12 for operation on gas. An increase from e = 7 to e = 10 will for instance result in a power increase of about 10%. One must, however, bear in mind that these engines have been designed for their original compression ratios with respect to the allowable load on the crankshaft bearings, etc. An increase of the compression ratio is furthermore an irreversible modification which does not allow operation of the engine with petrol any longer. Compression increase can be achived by machining off an appropriate portion of the cylinder head sealing surface. (For determination of the new compression volume refer to Chapter 5.5.3.) In some cases, however, the valves are very close to the piston and may touch the piston at TDC in the valve overlapping phase when the cylinder head is machined off.
With regard to the reasons given above the increase of compression ratio needs careful consideration and should rather be avoided with respect to engine life especially when the engine is earmarked for continuous operation. Otto (vehicle) engines are usually built for life spans of about 4 000 hours as opposed to diesel engines with life spans of 10 000 . . . 20 000 hours. The unavoidable power reduction in biogas operation should therefore be welcomed as a means to reduce wear and tear and increase the engine's life span.
6.5.1 Venturi Mixer
A venturi mixer in its details is given in Appendix IV. The body can be manufactured from a standard steel tube but should be somehow finished inside to obtain a smooth surface. The connecting flanges are made in accordance with the flange size of the engine's inlet manifold and air filter respectively. The venturi ring requires careful machining on a lathe machine and an extremely smooth surface. The ring groove around the circumference which forms the fuel gas channel to supply the gas inlet jets should have a free area of at least 1.5 times the total area of the jet bores to provide a slow flow with only little resistance.
The bore holes are to be evenly distributed around the circumference, the number of holes being chosen in such a way that the individual bore has a diameter of between 2 mm for smaller engines and 4 mm for larger engines respectively. The previously calculated fuel gas inlet area is divided by the number of holes to obtain the area of the individual bore Ab, its diameter db found by
(Equ. 6.5)
The outer diameter of the venturi ring is to be machined to precisely match with the inner diameter of the mixer body to avoid uncontrolled air bypass. An extra O-ring in a groove will suffice to tighten the venturi ring against the tube body. The venturi ring is held in position by a setscrew fitting into a small hole in the center of the circumferential fuel gas supply channel. The setscrew should not block any of the fuel gas bores and be positioned opposite the fuel gas supply pipe connection.
The fuel gas supply pipe from the plant should have a diameter large enough to keep the flow velocities lower than 2 m/s. In the normal case the use of a standard tube diameter is recommended, i.e. 3/8", 1/2", 3/4", etc., as the calibration valve can then be chosen from standard series also. The pipe can be brazed or welded into an appropriately sized hole drilled into the mixer body.
The choice of the calibration valve depends on the availability of technical equipment. Standard water valves made of brass may after some time show corrosion due to the H2S traces in the biogas but may be used where there is no alternative. Ball valves with stainless chicks are specifically recommended, also because they open and close with a 90° movement of their lever only and the optimum position can later be fixed with a stop screw easily.
The butterfly valve needs to be carefully manufactured in such a way that it can totally close the venturi mixer's flow area in the "closed" position. In any position it shall not interfere with the flow through the venturi ring. This means that its downstream distance from the venturi ring end needs to be at least 0.5 times the main channel's (inlet) diameter di. Some carburetor manufacturers choose to shape the butterfly valve as an ellipse so that it closes the flow channel at an angle smaller than 90° from the "open" position (see Fig. 6.2). This shape, however, is more difficult to obtain in a self-made version.
The two bearings holding the butterfly shaft require some precision in manufacturing. They need to
- allow free and easy movement of the shaft, especially when the butterfly valve is to be connected to an automatic control system,
- be airtight to prevent uncontrolled air to be sucked in and thus unbalance the calibration of the air/fuel ratio.
If the butterfly valve is operated manually and rarely only, rubber seals as shown in the detailed drawing can be used. For frequent and fine movement like in automatic control a brass or bronze bush on either side is more appropriate: Standard carburetors provide good examples also.
6.5.2 Use of Petrol Carburetors or Components
There are some reasons to furtherly utilize the original petrol carburetors in the process of air/fuel mixing:
- If the engine is to be operated on its original fuel in case of gas shortage the original carburetor can be retained completely and the gas mixer is mounted onto the carburetor's air inlet. In case of operation on fuel gas, petrol is no longer fed to the carburetor while fuel gas is fed to the mixer. A further advantage is that the butterfly valve of the carburetor is still used and the (venturi) mixer does not need its own butterfly valve In case of biogas shortage the gas supply is closed and the petrol supply opened. The ignition timing needs however to be readjusted whenever the type of fuel is changed (about 10° ... 15° earlier for biogas operation). Operation on the two fuels at one time is impossible as each individual mixer i.e. carburetor and gas mixer, is calibrated for single fuel operation only. The air/fuel mixture would become too rich.
- If petrol fuel shall not be used any longer the carburetor can still be retained to make use of the butterfly valve. In order to reduce the flow resistance by the carburetor its original venturi ring may even be removed together with the petrol inlet nozzle.
- Another alternative is the modification of the carburetor itself to act as a venturi gas mixer. This can be achieved by replacing the original carburetor venturi by a new one for biogas which has been designed and dimensioned according to the procedure in the previous chapter. A hole will have to be drilled into the carburetor body at a suitable place to insert the biogas supply pipe in such a way that it meets the internal ring channel of the venturi ring. The biogas supply pipe or a short tube for connection to a flexible hose pipe will have to be threaded if the carburetor body's wall thickness allows for screwing in. Otherwise a two-component epoxy resin glue can serve the purpose unless aluminum welding facilities are available.
- The original carburetor can also be modified by simply removing the petrol inlet nozzles and drilling one hole from the outside through the body and the original venturi. The hole will have to meet the venturi at its bottleneck and be big enough to allow the required biogas to join the airstream. The calculations need to consider the actual size of the given venturi (measure!) and the fact that the fuel gas is supplied by one inlet only.
The last alternative may be easier to manufacture but may also show inferior mixing qualities in cases where the distance to the manifold is short. The fuel gas emerging from one inlet (off-canter) may not have mixed sufficiently well with air before the mixture is distributed to the different cylinders. Individual cylinders may thus receive different mixtures which is unfavorable for uniform running.
The installation of the mixing chamber whether on an existing carburetor or air filter or in the place of the previous carburetor follows the same guidelines as given in Chapter 5.
6.6.1 Manual Mode
The only control device of an Otto engine is the butterfly valve which varies the amount of combustible air/fuel mixture admitted to the cylinder. The speed or power output of the engine can therefore only be controlled by opening and closing the butterfly valve. If the valve is kept in one position and the load drawn from the engine drops, the engine will increase speed until speed and load have found a new balance. If the load is too low to find a new balance the engine overspeeds and can finally destroy itself. In case of load increase the speed of the engine decreases. If the load drawn from the engine does not decrease also the engine can finally come to a standstill. In case of a new balance of load and speed the engine continues operation at lower speed which may be hazardous at speeds below about 1300 1/min when operating at high load for longer periods.
Manual operation therefore requires the presence of an experienced operator to take care of load fluctuations and operate the butterfly valve accordingly. Unlike diesel engines Otto engines have no overspeed. safety device or governor in most cases. Some however are equipped with a simple centrifugal mechanism within their distributor rotor which cuts out the ignition at any speed above the maximum. The engine is not shut off completely but continues with speed fluctuating around the maximum in an "on and off" mode which should not be tolerated for more than a few minutes.
6.6.2 Automatic Mode
The principal methods for automatic operation and control have already been dealt with in greater detail in Chapter 5 on diesel gas engines.
Fig. 6.8: Mechanical speed governor
directly acting on the butterfly throttle in the mixing valve.
Some Otto engines originally designed for stationary purposes may have a centrifugal governor type of control mechanism which can of course be used. The motion of the governor rack or lever will have to operate the butterfly valve of the new mixer, whether venturi or mixing valve, or it simply continues to operate the former carburetor's butterfly if the latter is still used in a modified form. Electronic systems which sense the engine speed and operate the butterfly valve with a positioner may be easier to install at an engine that has no shaft connection for a mechanical governor.
Electronic control systems are, however, sensitive to rough climate and handling and need suitable expertise for maintenance and repair. A separate mechanical governor, even if driven by a V-belt from the crankshaft pulley, may appear to be more appropriate in cases where a little deviation (+ 3%) from the set speed is permissible.
An additional and separately connected overspeed device is always recommendable whether for manual or for automatic operation. The device, e.g. similar to an engine speed tachometer, interrupts the ignition circuit or energizes a solenoid valve in the gas supply line to make sure that the engine never runs at a speed higher than allowable. The engine should only be restarted by an operator who has carefully checked the reason for overspeeding, rectified the fault, and manually reset the system. 7. Planning a Biogas Engine System
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