Analysis of the fuel economy potential of a direct injection spark ignition engine and a CVT in an HEV and a conventional vehicle based on in-situ measurements

A Toyota OPA was selected as a test vehicle as it has the components of interest: a Direct Injection Spark Ignition (DISI) engine and a Continuously Variable Transmission (CVT). In order to estimate the benefit of the DISI engine and CVT, a 2001MY Toyota OPA was tested to collect the engine and CVT maps using in-situ measurement techniques. Two torque sensors were installed into the powertrain in the vehicle for that purpose; one is between the engine and transmission and the other one is installed on the driveshaft. The overall efficiency of the engine and transmission was estimated using the measured torques and speeds during Phase 3 of the FTP cycle The overall efficiencies of the engine at different operating modes including the lean and stoichiometric combustion modes were compared to each other. The overall efficiencies of the CVT are analyzed similarly. Finally, the measured steady state efficiency maps and emissions maps were used to predict the fuel economy and emissions of an HEV with the DISI engine and CVT. The FTP test for the test vehicle shows that Toyota has made a remarkable improvement of tailpipe HC and NOx emissions with their second generation DISI engine. The reduction of HC emissions is attributed to the improvement in the combustion system using a slit nozzle injector. The dominant factor for NOx reduction turns out to be the catalyst efficiency. Due to the increase in the catalyst capacity, the average catalyst efficiency for NOx is improved from 67.5% to 89.9%. The steady state characteristics of the DISI engine and CVT were collected successfully using the in-situ mapping technique. The operating range of the lean combustion was revealed. The maximum engine load for lean operation was 6 bar BMEP and the maximum engine speed was 2750 rpm. The improvement in steady state fuel consumption is about 20% at low speed and around 3 bar BMEP. The engine-out HC emissions are 2~3 times more and the engine-out NOx emissions are one-half to one-sixth of that in stoichiometric combustion mode. The energy losses were calculated from the measured power flows. The engine, the largest energy sink, consumes 62.3% of total energy loss during power mode and additionally consumes 11.8% more during idling and braking. The CVT consumes 5.6% and the vehicle consumes 20.2%. The overall efficiency of the engine, which is 29.3% during the Hot 505 cycle, is improved to 32.7% with the change in combustion mode to lean combustion. The resulting fuel economy improvement was measured as 5.7%. Therefore, it can be concluded that the fuel economy benefit of the second generation Toyota DISI engine over a PFI engine during Phase 3 of the FTP cycle is 5.7% which is due to the 3.4% improvement in the overall engine efficiency. The benefit of a DISI engine over a conventional SI engine in an HEV application is found to be 4.2% in terms of composite fuel economy and 3.9% for the Hot 505 cycle, which is less than that of 5.7% for a DISI engine in a conventional vehicle. The overall engine efficiency improvement of a DISI engine in an HEV application is 0.5 percentage points which is also less than that for a DISI engine in a conventional vehicle application. This is because the engine is working in the high load region due to the down-sized engine. This DISI engine operates primarily in homogeneous charge mode for high load, and thus does not offer a large fuel economy benefit. The HC emissions of both types of engines are similar to each other and the NOx emissions of HEV with a DISI engine is 26% higher than that with an SI engine.