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3.5 Exhaust-gas measurements on 2-stroke engines
3.5.1 Engineering
Vehicles with a 2-stroke engine have higher hydrocarbon (HC)
emissions than vehicles with a 4-stroke engine and they also emit
oil. Oil consists primarily of hydrocarbons. If no precautions are
taken to prevent this, they are deposited on the walls of the outer
gas path (probe, hose and filter).
These deposits result in an HC concentration reading (residual-
value reading) even when no exhaust-gas measurement is con-
ducted, i.e. the actual value in an HC measurement is corrupted
(increased).
This effect, referred to as "hang-up" in specialist circles, occurs
with all exhaust-gas measuring instruments irrespective of make.
It is only visible in the case of equipment with HC measurement.
The majority of these deposits can be prevented by using
activated charcoal filters. These filters bind and neutralise most
of the oil and volatile hydrocarbons. Such filters have a limited
service life. They are to be installed in the gas path on the probe
downstream of the coarse filter.
Small quantities of deposits may form on the hose walls downst-
ream of the activated charcoal filter. They must be removed. We
suggest two alternative methods:
3.5.2 Solutions
Alternative 1
Flushing using the pump integrated into the test equipment.
After each measurement of a two-stroke engine, the pump must
remain on until the indicated HC value has dropped below 20
ppm. The length of the flushing time depends on how high the
residual value is. It may be about 30 minutes or considerably
longer.
For this alternative, the only extra item required to measure the
exhaust-gas emissions of a two-stroke engine is the activated
charcoal filter. It must always be installed in the gas path for these
measurements.
1. Coarse filter 0 450 904 058
2. Activated charcoal filter 1 687 014
3. Sampling hose of polyurethane
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BOSCH
Made in USA
459666/3
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Alternative 2
To reduce the flushing times drastically, i.e. to increase the
availability of the equipment after two-stroke measurement, we
recommend a second external gas path.
Possible deposits are quickly removed by blowing out the hose
with compressed air.
! Disconnect the hose from the instrument.
Do not blow compressed air into the instrument.
1. Coarse filter 1 687 432 005
2. Activated charcoal filter 1 687 432 014
3. Sampling hose of polyurethane
4. Sampling-hose set 1 687 001 283
made of silicone, length (7.5 + 0.3 + 0.3)m
3.5.3 Notes
– The sampling hose for the 2nd gas path is made of silicone.
Compared with the hose made of polyurethane, it is less
expensive, but it absorbs hydrocarbons.
– Activated charcoal filters bind hydrocarbons.
! The silicone sampling hose and activated charcoal filter may
only be used for CO measurements, not for HC mea-
surements.
BOSCH
Made in USA
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459666/4
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