Exhaust Analysis On 2-Stroke Engines - Bosch ETT 8.70 Instrucciones De Manejo

3.6

Exhaust analysis on 2-stroke engines

3.6.1 Technical background
Vehicles with 2-stroke engines emit higher levels of hydrocarbon
(HC) emissions than 4-stroke engines and they also emit oil. Oil
for the most part consists of hydrocarbons. If no measures are
taken to prevent it from happening, this oil is deposited on the
sides/walls of the external gas path (sensor, hose, filter).
These deposits lead to an HC concentration being indicated
(residual value indication) even when exhaust analysis is not being
carried out. This means, then, that the actual HC value measured
during an HC measurement is distorted by the amount of this
residual value (increased).
This effect, referred to by experts as "hang-up", manifests itself in
all exhaust-gas analysers that are capable of measuring the HC
concentration and is not specific to any particular make.
These deposits can be prevented to the greatest possible extent
through the use of activated charcoal filters. These filters bind and
neutralise for the most part oil and volatile hydrocarbons. Filters
of this kind have a limited service life. They are used on the sensor
in the gas path downstream of the coarse filter.
Deposits can also occur in small quantities downstream of the
activated charcoal filter on the sides of the hose. They must be
purged by means of one or another of the two alternatives we
offer.
3.6.2 Solutions
1st alternative
Flushing using the integral pump in the analyser.
The pump must be left switched on after each 2-stroke measure-
ment until the HC value displayed has dropped to below 20 ppm.
This takes place in the respective exhaust-gas test step of the test
system being used. The flushing time depends on the magnitude
of the residual value. It can take approx. 30 minutes, but it may also
take considerably longer.
During this time, the exhaust-gas test step must not be exited.
The sampling probe must not be positioned in the exhaust pipe
while this is taking place.
For this alternative, only the activated charcoal filter is additionally
required for 2-stroke exhaust analysis. The filter must always be
inserted in the gas path during these measurements.
1. Coarse filter 0 450 904 058
2. Activated charcoal filter 1 687 432 014
3. Sampling hose, viton
2nd alternative
In order to reduce the flushing times drastically, i.e. to increase the
availability of the units after a 2-stroke analysis, we recommend
that you use a second external gas path.
The HC deposits can then be purged quickly by blowing out the
hose with compressed air.
1. Coarse filter 1 687 432 005
2. Activated charcoal filter 1 687 432 014
3. Sampling hose, viton
4. Sampling hose set 1 687 001 283,
silicone, 7.5 + 0.3 + 0.3 m long
3.6.3 Notes
- The sampling hose for the 2nd gas path is made of silicone.
It is cheaper than the hose made of viton, but is absorbs
hydrocarbons.
- Activated charcoal filters bind hydrocarbons.
- The silicone sampling hose and activated charcoal filter must
only be used for CO measurements, not for HC and lambda
measurements.
1 2 3
4
1 2
1 3
459571/4P
459571/3P
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