Appendix III

Comparative Summary of Engine Features

Feature	Diesel	Otto
1. Design data
- compression ratio e	15 . . . 21	6 . . . 9.5 petrol
			6 .. .12 alcohol
- pressure after compression
without ignition	35 . . . 60 bar	15 . . . 20 bar
- temperature after compres-
sion without ignition	600. . . 900 °C	400 . . .600 °C
-excess air ratio l	1.3 . . . 4.0	0.7 . . .1.2
-efficiency	0.3 . . . 0.4	0.2. . . 0.35
-specific fuel consumption	230 . . . 350 g/kWh	300 . . .400 g/kWh
-volumetric efficiency	0.7 . . . 0.9	0.3 . . . 0.9 (low values for
			partially closed throttle)
-exhaust gas temperature	400 . . . 600 °C	500 . . . 900 °C
- speed ratio
- stationary	1,300 . . . 2,500	1,300 . . . 2,500 (gas)
	- vehicle	1,300 . . .5,000	1,300 . . . 7,000
	- ignition type	self-ignition by injection of fuel into hot compressed air shortly before piston reaches TDC	spark ignition by spark plug
	2. Control principle	variation of amount of fuel injected by the injector pump. Airflow is not controlled, i.e. full compression is always achieved. The variation of amount of fuel is done by the centrifugal mechanism of the governor with the aim to maintain the speed chosen and set by the control lever position.	variation of admission of ready air/fuel mixture by throttle valve between mixing device (carburetor, venturi, mixing valve) and engine inlet. Throttling reduces actual suction pressure of engme, hence absolute compression and efficiency.
	- manual	by setting the governor control lever to the required speed which remains constant within small limits, irrespective of the actual power demand. Speed changes can be achieved by setting the lever to a different position.	by setting the lever of the butterfly valve (throttle) in the carburetori load/speed variations require an appropriate regulation of the throttle.
	- automatic	using the same mechanism as above.	- using a separately mounted mechanical governor to operate the throttle.
			- using an electronic speed sensor with control unit and actuator to operate the throttle.

Features of Biogas Engines
(only where different from unmodified engines)

Feature	Gas diesel	Gas Otto
1. Design data
-compression ratio e	15 . . .18	10. . . 12
-excess air ratio	1.3 . . . 4.0	0.9 . . . 1.3
-specific fuel consumption	0.55 . . . 0.75 m³/kWh	0.65 . . .1.0 m³/kWh
	(+ pilot fuel)
-exhaust gas temperature	500 . . . 700 °C	500 . . . 900 °C
- ignition type	self-ignition of pilot fuel inject- ed into a hot compressed mix ture of air and gas which is ignited by the pilot fuel sub sequently.	as in other Otto engines
2. Control principle	A small amount of diesel fuel is injected to facilitate ignition. Variation of the amount of fuel gas supplied to the mixing de vice is used for variation of power output. The airflow is not con trolled to maintain a high pres sure and ignition temperature. mode	as in other Otto engines
- manual	The governor/injector system is fixed at supplying the pilot fuel amount only. The gas valve at the mixing chamber is set to achieve the required speed/power output.	as in other Otto engines. Carburetor is replaced by venturi mixer or gas mixing valves.
- automatic	Using the same mechanism as above. The gas valve is however operated by a governor or an actuator of an electronic control system.	as in other Otto engines.
Overview of Mode of Operation, Control and Mixing Device
Mode of operation	Type of control	Type of mixing device
speed: constant load: constant (e.g. pump with constant head and capacity; electric generator with constant load and frequency)	- manual
	Otto: fixed setting of gas and air or throttle in the case of a venturi	Otto: - simple mixing chamber with manually operated control valve for air and for gas, or
		- venturi-type mixer with manu diesel:
	fixed setting of gas	- simple mixing chamber with manually operated control valve
		- automatic not necessary as long as load remains constant
speed: constant load: varying (e.g. electric generator with constant frequency and varying electricity demand; pump with varying capacity and head)	- manual
	Otto: adjustment of gas/air valves or throttle (venturi) whenever load changes. Not recommended with frequent load changes.	Otto:
		- venturi mixer with manually operated throttle (Simple mixing chamber with two valves is impracticable for readjustments.)
		- mixing valve with manually operated butterfly throttle diesel:
	diesel:
	adjustment of gas valve whenever load changes. Without adjustment load variations are compensated by variations in diesel fuel supply automatic ally. Substitution of diesel fuel by biogas is however reduced.	- simple mixing chamber with manually operated gas valve
	- automatic
	Otto:	Otto:
	speed governor or electronic control system operating the butterfly throttle of mixing device diesel: fixed setting of pilot fuel injection. Electronic control or governor operating the gas valve of the mixing chamber.	- venturi mixer, or
		- mixing valve with butterfly valve operated by control system
		diesel:
		- simple mixing chamber with gas valve operated by control system
speed: varying load: varying (e.g. drive of different machinery)	- manual
	Otto: adjustment of throttle valve in accordance with required load/speed	Otto: - venturi mixer, or - gas mixing valve with manually operated butterfly
	diesel:	diesel:
	adjustment of gas valve in accordance with required load/speed	- simple mixing chamber with manually operated gas valve
	- automatic
	Otto:	Otto:
	mechanical governor or electronic control system with practicable mode of set point adjustment	- venturi mixer, or - gas mixing valve with butterfly valve operated by control system
	diesel:	diesel:
	fixed setting of pilot fuel injection. Electronic control system or (separate) speed governor with	- simple mixing chamber with gas valve operated by control system practicable mode of set point adjustment.