
                                                                    Exhibit 4f


                [CLEAN DIESEL TECHNOLOGIES, INC. LETTERHEAD]

Dear Shareholder:

You will find enclosed the Preliminary  Prospectus (the  "Prospectus")  and
other materials  relating to the rights offering (the "Rights Offering") by
Clean  Diesel  Technologies,  Inc.  ("CDT").  Please  carefully  review the
enclosed  Prospectus,  including the sections titled "The Rights Offering,"
which describes how you may participate in the Rights Offering, "Business,"
which more fully describes the Company's business, "Management's Discussion
and Analysis of Financial  Condition and Results of Operations," which more
fully describes the Company's financial position, and "Risk Factors," which
describes  certain risks associated with an investment in securities of the
Company, as well as the same sections in the final Prospectus which will be
mailed  after the  Registration  Statement  relating to the  prospectus  is
declared effective by the Securities and Exchange  Commission.  Capitalized
terms used but not described herein have the respective  meanings set forth
in the Prospectus.

The  following is a brief  review of our products and markets,  and current
financial situation.

When we formed CDT, we had patents for  Platinum  Fuel  Catalysts  ("PFCs")
licensed from Fuel Tech and limited testing results of PFCs. We now have 12
US  and 38  International  patents  on  PFCs  and  nitrogen  oxide  ("NOx")
reduction  systems and have  another 83 US and  International  applications
pending.  In addition,  the Company has completed  successful  testing of a
diesel  fuel  PFC  additive,  launched  the  marketing  of a  PFC  used  to
rejuvenate aged catalytic  converters in Europe, and developed the Advanced
Reagent  Injection  System  ("ARIS(TM)  2000")  for  use in  catalytic  NOx
reduction systems.

The  Company  has  successfully   completed  a  study  at  Delft  Technical
University in the  Netherlands  to determine the  fundamental  mechanism of
metallic additives. In particular, we investigated the function of platinum
as part of a composite additive with other catalysts.  Results of the study
show that a system of a bimetallic  additive of platinum and cerium (Pt/Ce)
with a ceramic filter:

     *    gives performance superior to any system known,

     *    gives a significantly lower oxidation temperature for soot, and

     *    uses  much  lower  metal  levels  in the fuel  than  other  known
          additive filter systems.

The  Delft  paper  has  provoked  excellent  market  response  from  engine
companies and catalyst system companies. The results reported by Delft were
confirmed  in  demonstrations  at  Cummins  Engine  Company  by a  test  in
Switzerland.

While the foregoing  results refer to a system  incorporating  filters,  we
have demonstrated  that the Pt/Ce additive also gives significant  emission
reductions on engines without particulate  filters or oxidizers.  Moreover,
the  emission   benefits  are  usually   accompanied   by  a  fuel  economy
improvement.

We now have to complete  registration  of the combined  Pt/Ce additive with
the EPA in order to be able to sell the  combined  additive for on-road use
in the U.S. We expect to complete the registration by October of 1998.

In the US, we believe there is a near term market  developing for additives
for diesel fuel which will reduce smoke, gaseous and particulate  emissions
from the current  fleet of  engines.  The market is a result of pending EPA
retrofit  guidelines which provide for State  Implementation  Plans ("SIP")
credits,  as well as state programs for  enforcement  of smoke  regulation,
which include  random  highway smoke tests with  substantial  penalties for
failure.

We believe  that another  market for Pt/Ce is in Taiwan,  where a bus fleet
trial confirmed the performance of the Pt/Ce filter system.

We believe  that a medium  term  market for Pt/Ce is with  filters  for new
vehicles,  especially light duty vehicles.  Peugeot has announced they will
introduce filters in 2000.

Using the Pt/Ce  additives in vehicles with filters  presents a potentially
very large market. This is driven by increasing concern about fine particle
emissions;  whereas new engines  produce little smoke,  the total number of
particles has increased.  Management  believes that filters are seen as the
preferred  remedy capable of removing more than 90% of the particles.  This
focus is given further weight by the probable  classification by California
of diesel  emissions as toxic.  The toxicity is most  associated  with soot
particles.  Once  collected  on the  filter,  the soot must be  oxidized to
prevent the filter from clogging, and that is where the additive is needed.

Our second product is a gasoline  additive which rejuvenates aged catalytic
converters.  This work was jointly funded by Holt Lloyd  International Ltd.
("Holts"),  which  launched  the product in the UK in 1997 in the  consumer
aftermarket  through auto parts  stores.  The results  were  disappointing.
Holts concluded that consumers did not understand what the product does and
the  majority  did not know if their cars had  catalytic  converters.  Test
marketing  by Holts  concluded  that the  product  should  be sold  through
service and repair shops. Holts is considering how to better reposition the
product.

In the US, we have  interest from  marketing  companies to package and sell
the gasoline  product.  We are seeking a marketing partner who will fund or
jointly fund the necessary  registration and  demonstration of this product
in the US. The  product may be launched in the US during the second half of
1999, although no assurances can be given in this regard.

Our third  product is for NOx  reduction.  We have  developed,  built,  and
tested a  prototype  system  called the  ARIS(TM)  2000  (Advanced  Reagent
Injection System).  The ARIS system injects urea over a catalyst to achieve
NOx reduction of up to 90%. The catalysts are similar to catalysts  used in
gas turbines and boilers and are made by catalyst manufacturers. We plan to
commercialize this system through cooperative ventures and licenses.

Current  regulation  in  California  and the  Northeast  states  for  large
stationary  diesel  engines sets NOx levels at a 50% to 90% reduction  from
normal levels  achieved  from current  production  engines under  federally
mandated Best Available Control  Technology  ("BACT") and Lowest Achievable
Emission Rate ("LAER")  requirements in ozone  non-attainment  areas.  Cost
effective systems are not yet available. As a result, gas engines are being
installed,  which cost more to buy and to run than the ARIS(TM)  2000.  The
Company intends to sell the ARIS(TM) 2000 to total system suppliers, engine
companies,  engine  distributors  or the  assemblers  of  generation  sets,
compressors, pumps, etc.

In summary, we started with a  partially-developed  diesel additive with no
defined market and a concept for a NOx reduction system.

     *    We now have  developed a diesel PFC  additive  which is effective
          both  for  emission  reduction  from  engines  and  significantly
          improves the performance of filters.

     *    We have launched the  marketing of a PFC used to rejuvenate  aged
          catalytic converters in Europe.

     *    We have  developed  the ARIS(TM)  2000 for use in  catalytic  NOx
          reduction systems.

Through the net  proceeds  of the Rights  Offering,  together  with the net
proceeds from the recent bridge  financing,  CDT seeks to commercialize the
foregoing products.

If you have any questions concerning the Rights Offering,  please feel free
to  telephone  the  Information  Agent at ( )          . On  behalf  of the
Board of Directors, we thank you for your support and confidence  and  look
forward to continuing to serve you.

                                          Sincerely,


                                          Jeremy D. Peter-Hoblyn
                                          President and Chief Executive
                                          Officer, Director

THIS  LETTER  CONTAINS  FORWARD-LOOKING  STATEMENTS  WITHIN THE  MEANING OF
SECTION   27A  OF  THE   SECURITIES   ACT.   DISCUSSION   CONTAINING   SUCH
FORWARD-LOOKING  STATEMENTS  WILL BE FOUND IN THE  MATERIAL SET FORTH UNDER
"PROSPECTUS  SUMMARY,  "RISK  FACTORS,"  "USE OF  PROCEEDS,"  "MANAGEMENT'S
DISCUSSION  AND ANALYSIS OF FINANCIAL  CONDITION AND RESULTS OF OPERATIONS"
AND "BUSINESS," AS WELL AS WITHIN THE PROSPECTUS GENERALLY.  ACTUAL RESULTS
COULD  DIFFER  MATERIALLY  FROM  THOSE  PROJECTED  IN  THE  FORWARD-LOOKING
STATEMENTS  AS A RESULT OF THE RISK FACTORS SET FORTH UNDER "RISK  FACTORS"
AND THE MATTERS SET FORTH IN THIS PROSPECTUS GENERALLY.

A REGISTRATION  STATEMENT  RELATING TO THESE SECURITIES HAS BEEN FILED WITH
THE  SECURITIES AND EXCHANGE  COMMISSION BUT HAS NOT YET BECOME  EFFECTIVE.
THE  SECURITIES  MAY NOT BE SOLD NOR MAY OFFERS TO BUY BE ACCEPTED PRIOR TO
THE TIME THE REGISTRATION  STATEMENT BECOMES  EFFECTIVE.  THIS LETTER SHALL
NOT CONSTITUTE AN OFFER TO SELL OR THE  SOLICITATION OF AN OFFER TO BUY NOR
SHALL THERE BE ANY SALE OF THE SECURITIES IN ANY JURISDICTION IN WHICH SUCH
OFFER,  SOLICITATION  OR SALE WOULD BE UNLAWFUL  PRIOR TO  REGISTRATION  OR
QUALIFICATION UNDER THE SECURITIES LAWS OF ANY JURISDICTION.

THE USE OF THIS LETTER IS AUTHORIZED ONLY WHEN PRECEDED OR ACCOMPANIED BY A
PROSPECTUS.
